OCTOBER TERM, 2000

															Syllabus

															KYLLO v. UNITED STATES

															CERTIORARI TO THE UNITED STATES COURT OF APPEALS FOR THE NINTH
CIRCUIT

															No. 99-8508. Argued February 20, 200l-Decided June 11,2001

															Suspicious that marijuana was being grown in petitioner Kyllo's
home in a triplex, agents used a thermal-imaging device to scan the
triplex to determine if the amount of heat emanating from it was
consistent with the high-intensity lamps typically used for indoor
marijuana growth. The scan showed that Kyllo's garage roof and a
side wall were relatively hot compared to the rest of his home and
substantially warmer than the neighboring units. Based in part on
the thermal imaging, a Federal Magistrate Judge issued a warrant to
search Kyllo's home, where the agents found marijuana growing.
After Kyllo was indicted on a federal drug charge, he
unsuccessfully moved to suppress the evidence seized from his home
and then entered a conditional guilty plea. The Ninth Circuit
ultimately affirmed, upholding the thermal imaging on the ground
that Kyllo had shown no subjective expectation of privacy because
he had made no attempt to conceal the heat escaping from his home.
Even if he had, ruled the court, there was no objectively
reasonable expectation of privacy because the thermal imager did
not expose any intimate details of Kyllo's life, only amorphous hot
spots on his home's exterior. Held: Where, as here, the Government uses a device that
is not in general public use, to explore details of a private home
that would previously have been unknowable without physical
intrusion, the surveillance is a Fourth Amendment "search," and is
presumptively unreasonable without a warrant. Pp. 31-41.

															(a) The question whether a warrantless search of a home is
reasonable and hence constitutional must be answered no in most
instances, but the antecedent question whether a Fourth Amendment
"search" has occurred is not so simple. This Court has approved
warrantless visual surveillance of a home, see California v. Ciraolo, 476
U. S. 207 , 213, ruling that visual observation is no "search"
at all, see Dow Chemical Co. v. United States, 476 U. S. 227 ,
234-235, 239. In assessing when a search is not a search, the Court
has adapted a principle first enunciated in Katz v. United States, 389 U. S. 347 , 361: A
"search" does not occur-even when its object is a house explicitly
protected by the Fourth Amendment-unless the individual manifested
a subjective 28 Syllabus

															expectation of privacy in the searched object, and society is
willing to recognize that expectation as reasonable, see, e. g.,
California v. Ciraolo, supra, at 211. Pp. 31-33.

															(b) While it may be difficult to refine the Katz test in
some instances, in the case of the search of a home's interior-the
prototypical and hence most commonly litigated area of protected
privacy-there is a ready criterion, with roots deep in the common
law, of the minimal expectation of privacy that exists, and
that is acknowledged to be reasonable. To withdraw
protection of this minimum expectation would be to permit police
technology to erode the privacy guaranteed by the Fourth Amendment.
Thus, obtaining by sense-enhancing technology any information
regarding the home's interior that could not otherwise have been
obtained without physical "intrusion into a constitutionally
protected area," Silverman v. United States, 365 U. S. 505 , 512,
constitutes a search-at least where (as here) the technology in
question is not in general public use. This assures preservation of
that degree of privacy against government that existed when the
Fourth Amendment was adopted. Pp. 33-35.

															(c) Based on this criterion, the information obtained by the
thermal imager in this case was the product of a search. The Court
rejects the Government's argument that the thermal imaging must be
upheld because it detected only heat radiating from the home's
external surface. Such a mechanical interpretation of the Fourth
Amendment was rejected in Katz, where the eavesdropping
device in question picked up only sound waves that reached the
exterior of the phone booth to which it was attached. Reversing
that approach would leave the homeowner at the mercy of advancing
technology-including imaging technology that could discern all
human activity in the home. Also rejected is the Government's
contention that the thermal imaging was constitutional because it
did not detect "intimate details." Such an approach would be wrong
in principle because, in the sanctity of the home, all details are intimate details. See, e. g., United States v. Karo, 468 U. S. 705; Dow Chemical, supra, at 238,
distinguished. It would also be impractical in application, failing
to provide a workable accommodation between law enforcement needs
and Fourth Amendment interests. See Oliver v. United
States, 466 U. S.
170 , 181. Pp. 35-40.

															(d) Since the imaging in this case was an unlawful search, it
will remain for the District Court to determine whether, without
the evidence it provided, the search warrant was supported by
probable cause-and if not, whether there is any other basis for
supporting admission of that evidence. P. 40. 190 F.3d
1041 , reversed and remanded. 29 SCALIA, J., delivered the opinion of the Court, in which SOUTER,
THOMAS, GINSBURG, and BREYER, JJ., joined. STEVENS, J., filed a
dissenting opinion, in which REHNQUIST, C. J., and O'CONNOR and
KENNEDY, JJ., joined, post, p. 41. Kenneth Lerner, by appointment of the Court, 531 U. S.
955, argued the cause and filed briefs for petitioner.

															Deputy Solicitor General Dreeben argued the cause for the United
States. With him on the brief were former Solicitor General Waxman,
Assistant Attorney General Robinson, Irving L. Gornstein, and
Deborah Watson. *

															JUSTICE SCALIA delivered the opinion of the Court.

															This case presents the question whether the use of a
thermal-imaging device aimed at a private home from a public street
to detect relative amounts of heat within the home constitutes a
"search" within the meaning of the Fourth Amendment.

															I

															In 1991 Agent William Elliott of the United States Department of
the Interior came to suspect that marijuana was being grown in the
home belonging to petitioner Danny Kyllo, part of a triplex on
Rhododendron Drive in Florence, Oregon. Indoor marijuana growth
typically requires highintensity lamps. In order to determine
whether an amount of heat was emanating from petitioner's home
consistent with the use of such lamps, at 3:20 a.m. on January 16,
1992, Agent Elliott and Dan Haas used an Agema Thermovision 210
thermal imager to scan the triplex. Thermal imagers detect infrared
radiation, which virtually all objects emit but which is not
visible to the naked eye. The imager converts radiation into images
based on relative warmth-black

															*Briefs of amici curiae urging reversal were filed for
the Liberty Project by Julie M. Carpenter; and for the
National Association of Criminal Defense Lawyers et al. by James J. Tomkovicz, Lisa B. Kemler, and Steven R.
Shapiro. 30 is cool, white is hot, shades of gray connote relative
differences; in that respect, it operates somewhat like a video
camera showing heat images. The scan of Kyllo's home took only a
few minutes and was performed from the passenger seat of Agent
Elliott's vehicle across the street from the front of the house and
also from the street in back of the house. The scan showed that the
roof over the garage and a side wall of petitioner's home were
relatively hot compared to the rest of the home and substantially
warmer than neighboring homes in the triplex. Agent Elliott
concluded that petitioner was using halide lights to grow marijuana
in his house, which indeed he was. Based on tips from informants,
utility bills, and the thermal imaging, a Federal Magistrate Judge
issued a warrant authorizing a search of petitioner's home, and the
agents found an indoor growing operation involving more than 100
plants. Petitioner was indicted on one count of manufacturing
marijuana, in violation of 21 U. s. C. § 841(a)(1). He
unsuccessfully moved to suppress the evidence seized from his home
and then entered a conditional guilty plea.

															The Court of Appeals for the Ninth Circuit remanded the case for
an evidentiary hearing regarding the intrusiveness of thermal
imaging. On remand the District Court found that the Agema 210 "is
a non-intrusive device which emits no rays or beams and shows a
crude visual image of the heat being radiated from the outside of
the house"; it "did not show any people or activity within the
walls of the structure"; "[t]he device used cannot penetrate walls
or windows to reveal conversations or human activities"; and "[n]o
intimate details of the home were observed." Supp. App. to Pet. for
Cert. 39-40. Based on these findings, the District Court upheld the
validity of the warrant that relied in part upon the thermal
imaging, and reaffirmed its denial of the motion to suppress. A
divided Court of Appeals initially reversed, 140
F.3d 1249 (1998), but that 31 opinion was withdrawn and the panel (after a change in
composition) affirmed, 190 F.3d
1041 (1999), with Judge Noonan dissenting. The court held that
petitioner had shown no subjective expectation of privacy because
he had made no attempt to conceal the heat escaping from his home,
id., at 1046, and even if he had, there was no objectively
reasonable expectation of privacy because the imager "did not
expose any intimate details of Kyllo's life," only "amorphous 'hot
spots' on the roof and exterior wall," id., at 1047. We granted
certiorari. 530 U. S. 1305 (2000).

															II

															The Fourth Amendment provides that "[t]he right of the people to
be secure in their persons, houses, papers, and effects, against
unreasonable searches and seizures, shall not be violated." "At the
very core" of the Fourth Amendment "stands the right of a man to
retreat into his own home and there be free from unreasonable
governmental intrusion." Silverman v. United States, 365 U. S. 505 ,
511 (1961). With few exceptions, the question whether a warrantless
search of a home is reasonable and hence constitutional must be
answered no. See Illinois v. Rodriguez, 497 U. S. 177 , 181
(1990); Payton v. New York, 445 U. S. 573 , 586
(1980).

															On the other hand, the antecedent question whether or not a
Fourth Amendment "search" has occurred is not so simple under our
precedent. The permissibility of ordinary visual surveillance of a
home used to be clear because, well into the 20th century, our
Fourth Amendment jurisprudence was tied to common-law trespass.
See, e. g., Goldman v. United States, 316 U. S. 129 , 134-136
(1942); Olmstead v. United States, 277 U. S. 438 , 464-466
(1928). Cf. Silverman v. United States, supra, at
510-512 (technical trespass not necessary for Fourth Amendment
violation; it suffices if there is "actual intrusion into a
constitutionally protected area"). Visual surveillance was
unquestionably lawful because" 'the 32 eye cannot by the laws of England be guilty of a trespass.'" Boyd v. United States, 116 U. S. 616 , 628 (1886)
(quoting Entick v. Carrington, 19 How. St. Tr. 1029,
95 Eng. Rep. 807 (K. B. 1765)). We have since decoupled violation
of a person's Fourth Amendment rights from trespassory violation of
his property, see Rakas v. III inois, 439 U. S. 128 , 143
(1978), but the lawfulness of warrantless visual surveillance of a
home has still been preserved. As we observed in California v. Ciraolo, 476 U. S. 207 , 213
(1986), "[t]he Fourth Amendment protection of the home has never
been extended to require law enforcement officers to shield their
eyes when passing by a home on public thoroughfares."

															One might think that the new validating rationale would be that
examining the portion of a house that is in plain public view,
while it is a "search" 1 despite the absence of trespass, is not an
"unreasonable" one under the Fourth Amendment. See Minnesota v. Carter, 525
U. S. 83 , 104 (1998) (BREYER, J., concurring in judgment). But
in fact we have held that visual observation is no "search" at
allperhaps in order to preserve somewhat more intact our doctrine
that warrantless searches are presumptively unconstitutional. See Dow Chemical Co. v. United States, 476 U. S. 227,
234-235, 239 (1986). In assessing when a search is not a search, we
have applied somewhat in reverse the principle first enunciated in Katz v. United States, 389 U. S. 347 (1967). Katz involved eavesdropping by means of an electronic
listening device placed on the outside of a telephone booth-a
location not within the catalog ("persons, houses, papers, and
effects") that the Fourth Amendment protects against unreasonable
searches. We held that the

															1 When the Fourth Amendment was adopted, as now, to "search"
meant "[tJo look over or through for the purpose of finding
something; to explore; to examine by inspection; as, to search the house for a book; to search the wood for a
thief." N. Webster, An American Dictionary of the English Language
66 (1828) (reprint 6th ed. 1989). 33 Fourth Amendment nonetheless protected Katz from the warrantless
eavesdropping because he "justifiably relied" upon the privacy of
the telephone booth. Id., at 353. As Justice Harlan's
oft-quoted concurrence described it, a Fourth Amendment search
occurs when the government violates a subjective expectation of
privacy that society recognizes as reasonable. See id., at 361. We
have subsequently applied this principle to hold that a Fourth
Amendment search does not occur-even when the explicitly
protected location of a house is concerned-unless "the
individual manifested a subjective expectation of privacy in the
object of the challenged search," and "society [is] willing to
recognize that expectation as reasonable." Ciraolo, supra, at 211. We have applied this test in holding that it is not a
search for the police to use a pen register at the phone company to
determine what numbers were dialed in a private home, Smith v. Maryland, 442 U. S. 735 , 743744
(1979), and we have applied the test on two different occasions in
holding that aerial surveillance of private homes and surrounding
areas does not constitute a search, Ciraolo, supra; Florida v. Riley, 488
U. S. 445 (1989).

															The present case involves officers on a public street engaged in
more than naked-eye surveillance of a home. We have previously
reserved judgment as to how much technological enhancement of
ordinary perception from such a vantage point, if any, is too much.
While we upheld enhanced aerial photography of an industrial
complex in Dow Chemical, we noted that we found "it
important that this is not an area immediately adjacent to a
private home, where privacy expectations are most heightened," 476
U. S., at 237, n. 4 (emphasis in original).

															III

															It would be foolish to contend that the degree of privacy
secured to citizens by the Fourth Amendment has been 34 entirely unaffected by the advance of technology. For example,
as the cases discussed above make clear, the technology enabling
human flight has exposed to public view (and hence, we have said,
to official observation) uncovered portions of the house and its
curtilage that once were private. See Ciraolo, supra, at
215. The question we confront today is what limits there are upon
this power of technology to shrink the realm of guaranteed
privacy.

															The Katz test-whether the individual has an expectation
of privacy that society is prepared to recognize as reasonable-has
often been criticized as circular, and hence subjective and
unpredictable. See 1 W. LaFave, Search and Seizure § 2.1(d), pp.
393-394 (3d ed. 1996); Posner, The Uncertain Protection of Privacy
by the Supreme Court, 1979 S. Ct. Rev. 173, 188; Carter,
supra, at 97 (SCALIA, J., concurring). But see Rakas,
supra, at 143-144, n.12. While it may be difficult to refine Katz when the search of areas such as telephone booths,
automobiles, or even the curtilage and uncovered portions of
residences is at issue, in the case of the search of the interior
of homes-the prototypical and hence most commonly litigated area of
protected privacythere is a ready criterion, with roots deep in the
common law, of the minimal expectation of privacy that exists, and that is acknowledged to be reasonable. To
withdraw protection of this minimum expectation would be to permit
police technology to erode the privacy guaranteed by the Fourth
Amendment. We think that obtaining by senseenhancing technology any
information regarding the interior of the home that could not
otherwise have been obtained without physical "intrusion into a
constitutionally protected area," Silverman, 365 U. S., at
512, constitutes a searchat least where (as here) the technology in
question is not in general public use. This assures preservation of
that degree of privacy against government that existed when the
Fourth Amendment was adopted. On the basis of this criterion,
the 35 information obtained by the thermal imager in this case was the
product of a search.2

															The Government maintains, however, that the thermal imaging must
be upheld because it detected "only heat radiating from the
external surface of the house," Brief for United States 26. The
dissent makes this its leading point, see post, at 41,
contending that there is a fundamental difference between what it
calls "off-the-wall" observations and "through-the-wall
surveillance." But just as a thermal imager captures only heat
emanating from a house, so also a powerful directional microphone
picks up only sound emanating from a house-and a satellite capable
of scanning from many miles away would pick up only visible light
emanating from a house. We rejected such a mechanical
interpretation of the Fourth Amendment in Katz, where the
eavesdropping device picked up only sound waves that reached the
exterior of the phone booth. Reversing that approach would leave
the homeowner at the mercy of advancing technologyincluding imaging
technology that could discern all human

															2 The dissent's repeated assertion that the thermal imaging did
not obtain information regarding the interior of the home, post, at 43, 44 (opinion of STEVENS, J.), is simply
inaccurate. A thermal imager reveals the relative heat of various
rooms in the home. The dissent may not find that information
particularly private or important, see post, at 43-44, 45,
4950, but there is no basis for saying it is not information
regarding the interior of the home. The dissent's comparison of the
thermal imaging to various circumstances in which outside observers
might be able to perceive, without technology, the heat of the
home-for example, by observing snowmelt on the roof, post, at 43-is quite irrelevant. The fact that equivalent information
could sometimes be obtained by other means does not make lawful the
use of means that violate the Fourth Amendment. The police might,
for example, learn how many people are in a particular house by
setting up year-round surveillance; but that does not make breaking
and entering to find out the same information lawful. In any event,
on the night of January 16, 1992, no outside observer could have
discerned the relative heat of Kyllo's home without thermal
imaging. 36 activity in the home. While the technology used in the present
case was relatively crude, the rule we adopt must take account of
more sophisticated systems that are already in use or in
development.3 The dissent's reliance on the distinction between
"off-the-wall" and "through-the-wall" observation is entirely
incompatible with the dissent's belief, which we discuss below,
that thermal-imaging observations of the intimate details of a home
are impermissible. The most sophisticated thermal-imaging devices
continue to measure heat "off-the-wall" rather than
"through-the-wall"; the dissent's disapproval of those more
sophisticated thermalimaging devices, see post, at 49, is an
acknowledgment that there is no substance to this distinction. As
for the dissent's extraordinary assertion that anything learned
through "an inference" cannot be a search, see post, at 44,
that would validate even the "through-the-wall" technologies that
the dissent purports to disapprove. Surely the dissent does not
believe that the through-the-wall radar or ultrasound technology
produces an 8-by-10 Kodak glossy that needs no analysis (i. e., the making of inferences). And, of course, the novel
proposition that inference insulates a search is blatantly contrary
to United States v. Karo, 468 U. S. 705 (1984),
where the police "inferred" from the activation of a beeper that a
certain can of ether was in the home. The police ac-

															3 The ability to "see" through walls and other opaque barriers
is a clear, and scientifically feasible, goal of law enforcement
research and development. The National Law Enforcement and
Corrections Technology Center, a program within the United States
Department of Justice, features on its Internet Website projects
that include a "RadarBased Through-the-Wall Surveillance System,"
"Handheld Ultrasound Through the Wall Surveillance," and a "Radar
Flashlight" that "will enable law enforcement officers to detect
individuals through interior building walls." www.nlectc.org/techproj/ (visited May 3, 2001). Some devices may emit low levels of
radiation that travel "through-the-wall," but others, such as more
sophisticated thermal-imaging devices, are entirely passive, or
"off-the-wall" as the dissent puts it. 37 tivity was held to be a search, and the search was held
unlawfu1.4

															The Government also contends that the thermal imaging was
constitutional because it did not "detect private activities
occurring in private areas," Brief for United States 22. It points
out that in Dow Chemical we observed that the enhanced
aerial photography did not reveal any "intimate details." 476 U.
S., at 238. Dow Chemical, however, involved enhanced aerial
photography of an industrial complex, which does not share the
Fourth Amendment sanctity of the home. The Fourth Amendment's
protection of the home has never been tied to measurement of the
quality or quantity of information obtained. In Silverman, for example, we made clear that any physical invasion of the
structure of the home, "by even a fraction of an inch," was too
much, 365 U. S., at 512, and there is certainly no exception to the
warrant requirement for the officer who barely cracks open the
front door and sees nothing but the nonintimate rug on the
vestibule floor. In the home, our cases show, all details
are intimate details, because the entire area is held safe from
prying government eyes. Thus, in Karo, supra, the only thing
detected was a can of ether in the

															4 The dissent asserts, post, at 44-45, n. 3, that we have
misunderstood its point, which is not that inference insulates a search, but that inference alone is not a
search. If we misunderstood the point, it was only in a good-faith
effort to render the point germane to the case at hand. The issue
in this case is not the police's allegedly unlawful inferencing,
but their allegedly unlawful thermal-imaging measurement of the
emanations from a house. We say such measurement is a search; the
dissent says it is not, because an inference is not a search. We
took that to mean that, since the technologically enhanced
emanations had to be the basis of inferences before anything inside
the house could be known, the use of the emanations could not be a
search. But the dissent certainly knows better than we what it
intends. And if it means only that an inference is not a search, we
certainly agree. That has no bearing, however, upon whether hi-tech
measurement of emanations from a house is a search. 38 home; and in Arizona v. Hicks, 480 U. S. 321 (1987), the
only thing detected by a physical search that went beyond what
officers lawfully present could observe in "plain view" was the
registration number of a phonograph turntable. These were intimate
details because they were details of the home, just as was the
detail of how warm-or even how relatively warm-Kyllo was heating
his residence.5

															Limiting the prohibition of thermal imaging to "intimate
details" would not only be wrong in principle; it would be
impractical in application, failing to provide "a workable
accommodation between the needs of law enforcement and the
interests protected by the Fourth Amendment," Oliver v. United States, 466 U. S. 170 , 181
(1984). To begin with, there is no necessary connection between the
sophistication of the surveillance equipment and the "intimacy" of
the details that it observes-which means that one cannot say (and
the police cannot be assured) that use of the relatively crude
equipment at issue here will always be lawful. The Agema
Thermovision 210 might disclose, for example, at what hour each
night the lady of the house takes her daily sauna and bath-a detail
that many would consider "intimate"; and a much more sophisticated
system might detect nothing more intimate than the fact that
someone left a closet light on. We could not, in other words,
develop a rule approving only that through-the-wall surveillance
which identifies objects no smaller than 36 by 36 inches, but would
have to develop a jurisprudence specifying which

															5The Government cites our statement in California v. Ciraolo, 476 U. S. 207 (1986), noting apparent agreement
with the State of California that aerial surveillance of a house's
curtilage could become "'invasive'" if "'modern technology'"
revealed "'those intimate associations, objects or activities
otherwise imperceptible to police or fellow citizens.'" Id., at 215, n. 3 (quoting Brief for State of California 14-15). We
think the Court's focus in this secondhand dictum was not upon
intimacy but upon otherwise-imperceptibility, which is precisely
the principle we vindicate today. 39 home activities are "intimate" and which are not. And even when
(if ever) that jurisprudence were fully developed, no police
officer would be able to know in advance whether his
through-the-wall surveillance picks up "intimate" detailsand thus
would be unable to know in advance whether it is
constitutional.

															The dissent's proposed standard-whether the technology offers
the "functional equivalent of actual presence in the area being
searched," post, at 47-would seem quite similar to our own
at first blush. The dissent concludes that Katz was such a
case, but then inexplicably asserts that if the same listening
device only revealed the volume of the conversation, the
surveillance would be permissible, post, at 49-50. Yet if,
without technology, the police could not discern volume without
being actually present in the phone booth, JUSTICE STEVENS should
conclude a search has occurred. Cf. Karo, 468 U. S., at 735
(STEVENS, J., concurring in part and dissenting in part) ("I find
little comfort in the Court's notion that no invasion of privacy
occurs until a listener obtains some significant information by use
of the device .... A bathtub is a less private area when the
plumber is present even if his back is turned"). The same should
hold for the interior heat of the home if only a person present in
the home could discern the heat. Thus the driving force of the
dissent, despite its recitation of the above standard, appears to
be a distinction among different types of information-whether the
"homeowner would even care if anybody noticed," post, at 50.
The dissent offers no practical guidance for the application of
this standard, and for reasons already discussed, we believe there
can be none. The people in their houses, as well as the police,
deserve more precision.6

															6 The dissent argues that we have injected potential uncertainty
into the constitutional analysis by noting that whether or not the
technology is in general public use may be a factor. See post, at 47. That quarrel, 40 We have said that the Fourth Amendment draws "a firm line at the
entrance to the house," Payton, 445 U. S., at 590. That
line, we think, must be not only firm but also brightwhich requires
clear specification of those methods of surveillance that require a
warrant. While it is certainly possible to conclude from the
videotape of the thermal imaging that occurred in this case that no
"significant" compromise of the homeowner's privacy has occurred,
we must take the long view, from the original meaning of the Fourth
Amendment forward. "The Fourth Amendment is to be construed in the light of what
was deemed an unreasonable search and seizure when it was adopted,
and in a manner which will conserve public interests as well as the
interests and rights of individual citizens." Carroll v. United States, 267 U. S. 132, 149 (1925). Where, as here, the Government uses a device that is not in
general public use, to explore details of the home that would
previously have been unknowable without physical intrusion, the
surveillance is a "search" and is presumptively unreasonable
without a warrant.

															Since we hold the Thermovision imaging to have been an unlawful
search, it will remain for the District Court to determine whether,
without the evidence it provided, the search warrant issued in this
case was supported by probable cause-and if not, whether there is
any other basis for supporting admission of the evidence that the
search pursuant to the warrant produced.

															however, is not with us but with this Court's precedent. See Ciraolo, supra, at 215 ("In an age where private and
commercial flight in the public airways is routine, it is
unreasonable for respondent to expect that his marijuana plants
were constitutionally protected from being observed with the naked
eye from an altitude of 1,000 feet"). Given that we can quite
confidently say that thermal imaging is not "routine," we decline
in this case to reexamine that factor. 41 ***

															The judgment of the Court of Appeals is reversed; the case is
remanded for further proceedings consistent with this opinion.

															It is so ordered.

															JUSTICE STEVENS, with whom THE CHIEF JUSTICE, JusTICE O'CONNOR,
and JUSTICE KENNEDY join, dissenting.

															There is, in my judgment, a distinction of constitutional
magnitude between "through-the-wall surveillance" that gives the
observer or listener direct access to information in a private
area, on the one hand, and the thought processes used to draw
inferences from information in the public domain, on the other
hand. The Court has crafted a rule that purports to deal with
direct observations of the inside of the home, but the case before
us merely involves indirect deductions from "off-the-wall"
surveillance, that is, observations of the exterior of the home.
Those observations were made with a fairly primitive thermal imager
that gathered data exposed on the outside of petitioner's home but
did not invade any constitutionally protected interest in privacy.l
Moreover, I believe that the supposedly "bright-line" rule the
Court has created in response to its concerns about future
technological developments is unnecessary, unwise, and inconsistent
with the Fourth Amendment.

															I

															There is no need for the Court to craft a new rule to decide
this case, as it is controlled by established principles from

															1 After an evidentiary hearing, the District Court found:

															"[T]he use of the thermal imaging device here was not an
intrusion into Kyllo's home. No intimate details of the home were
observed, and there was no intrusion upon the privacy of the
individuals within the home. The device used cannot penetrate walls
or windows to reveal conversations or human activities. The device
recorded only the heat being emitted from the home." Supp. App. to
Pet. for Cert. 40. 42 our Fourth Amendment jurisprudence. One of those core
principles, of course, is that "searches and seizures inside a
home without a warrant are presumptively unreasonable." Payton v. New York, 445 U. S. 573 , 586 (1980)
(emphasis added). But it is equally well settled that searches and
seizures of property in plain view are presumptively reasonable.
See id., at 586-587.2 Whether that property is residential or
commercial, the basic principle is the same: "'What a person
knowingly exposes to the public, even in his own home or office, is
not a subject of Fourth Amendment protection.'" California v. Ciraolo, 476 U. S. 207 , 213 (1986)
(quoting Katz v. United States, 389 U. S. 347 , 351
(1967)); see Florida v. Riley, 488 U. S. 445 , 449-450 (1989); California v. Greenwood, 486 U. S. 35 , 40-41
(1988); Dow Chemical Co. v. United States, 476 U. S. 227 , 235-236 (1986); Air Pollution Variance Bd. of Colo. v. Western
Alfalfa Corp., 416 U. S. 861 , 865
(1974). That is the principle implicated here.

															While the Court "take[s] the long view" and decides this case
based largely on the potential of yet-to-be-developed technology
that might allow "through-the-wall surveillance," ante, at
38-40; see ante, at 36, n. 3, this case involves nothing
more than off-the-wall surveillance by law enforcement officers to
gather information exposed to the general public from the outside
of petitioner's home. All that the infrared camera did in this case
was passively measure heat emitted

															2 Thus, for example, we have found consistent with the Fourth
Amendment, even absent a warrant, the search and seizure of garbage
left for collection outside the curtilage of a home, California v. Greenwood, 486 U. S. 35 (1988); the
aerial surveillance of a fenced-in backyard from an altitude of
1,000 feet, California v. Ciraolo, 476 U. S. 207 (1986); the
aerial observation of a partially exposed interior of a residential
greenhouse from 400 feet above, Florida v. Riley, 488 U. S. 445 (1989); the aerial photography of an industrial complex from
several thousand feet above, Dow Chemical Co. v. United
States, 476 U. S.
227 (1986); and the observation of smoke emanating from chimney
stacks, Air Pollution Variance Bd. of Colo. v. Western
Alfalfa Corp., 416 U. S. 861 (1974). 43 from the exterior surfaces of petitioner's home; all that those
measurements showed were relative differences in emission levels,
vaguely indicating that some areas of the roof and outside walls
were warmer than others. As still images from the infrared scans
show, see Appendix, infra, no details regarding the interior
of petitioner's home were revealed. Unlike an x-ray scan, or other
possible "throughthe-wall" techniques, the detection of infrared
radiation emanating from the home did not accomplish "an
unauthorized physical penetration into the premises," Silverman v. United States, 365 U. S. 505 , 509
(1961), nor did it "obtain information that it could not have
obtained by observation from outside the curtilage of the house," United States v. Karo, 468 U. S. 705 , 715
(1984).

															Indeed, the ordinary use of the senses might enable a neighbor
or passerby to notice the heat emanating from a building,
particularly if it is vented, as was the case here. Additionally,
any member of the public might notice that one part of a house is
warmer than another part or a nearby building if, for example,
rainwater evaporates or snow melts at different rates across its
surfaces. Such use of the senses would not convert into an
unreasonable search if, instead, an adjoining neighbor allowed an
officer onto her property to verify her perceptions with a
sensitive thermometer. Nor, in my view, does such observation
become an unreasonable search if made from a distance with the aid
of a device that merely discloses that the exterior of one house,
or one area of the house, is much warmer than another. Nothing more
occurred in this case.

															Thus, the notion that heat emissions from the outside of a
dwelling are a private matter implicating the protections of the
Fourth Amendment (the text of which guarantees the right of people
"to be secure in their ... houses" against unreasonable
searches and seizures (emphasis added)) is not only unprecedented
but also quite difficult to take seriously. Heat waves, like aromas
that are generated in a kitchen, or 44 in a laboratory or opium den, enter the public domain if and
when they leave a building. A subjective expectation that they
would remain private is not only implausible but also surely not
"one that society is prepared to recognize as 'reasonable.'" Katz, 389 U. S., at 361 (Harlan, J., concurring).

															To be sure, the homeowner has a reasonable expectation of
privacy concerning what takes place within the home, and the Fourth
Amendment's protection against physical invasions of the home
should apply to their functional equivalent. But the equipment in
this case did not penetrate the walls of petitioner's home, and
while it did pick up "details of the home" that were exposed to the
public, ante, at 38, it did not obtain "any information
regarding the interior of the home," ante, at 34
(emphasis added). In the Court's own words, based on what the
thermal imager "showed" regarding the outside of petitioner's home,
the officers "concluded" that petitioner was engaging in illegal
activity inside the home. Ante, at 30. It would be quite
absurd to characterize their thought processes as "searches,"
regardless of whether they inferred (rightly) that petitioner was
growing marijuana in his house, or (wrongly) that "the lady of the
house [was taking] her daily sauna and bath." Ante, at 38.
In either case, the only conclusions the officers reached
concerning the interior of the home were at least as indirect as
those that might have been inferred from the contents of discarded
garbage, see California v. Greenwood, 486 U. S. 35 (1988), or
pen register data, see Smith v. Maryland, 442 U. S. 735 (1979), or,
as in this case, subpoenaed utility records, see 190 F.3d
1041 , 1043 (CA9 1999). For the first time in its history, the
Court assumes that an inference can amount to a Fourth Amendment
violation. See ante, at 36-37.3

															3 Although the Court credits us with the "novel proposition that
inference insulates a search," ante, at 36, our point simply
is that an inference cannot be a search, contrary to the
Court's reasoning. See supra this page. Thus, the Court's
use of United States v. Karo, 468 U. S. 705 45 Notwithstanding the implications of today's decision, there is a
strong public interest in avoiding constitutional litigation over
the monitoring of emissions from homes, and over the inferences
drawn from such monitoring. Just as "the police cannot reasonably
be expected to avert their eyes from evidence of criminal activity
that could have been observed by any member of the public," Greenwood, 486 U. S., at 41, so too public officials should
not have to avert their senses or their equipment from detecting
emissions in the public domain such as excessive heat, traces of
smoke, suspicious odors, odorless gases, airborne particulates, or
radioactive emissions, any of which could identify hazards to the
community. In my judgment, monitoring such emissions with
"sense-enhancing technology," ante, at 34, and drawing
useful conclusions from such monitoring, is an entirely reasonable
public service.

															On the other hand, the countervailing privacy interest is at
best trivial. After all, homes generally are insulated to keep heat
in, rather than to prevent the detection of heat going out, and it
does not seem to me that society will suffer from a rule requiring
the rare homeowner who both intends to engage in uncommon
activities that produce extraordinary amounts of heat, and wishes
to conceal that production from outsiders, to make sure that the
surrounding area is well insulated. Cf. United States v. Jacobsen, 466
U. S. 109 , 122 (1984) ("The concept of an interest in privacy
that society is prepared to recognize as reasonable is, by its very
nature, critically different from the mere expectation, however
well

															(1984), to refute a point we do not make underscores the fact
that the Court has no real answer (either in logic or in law) to
the point we do make. Of course, Karo itself does not
provide any support for the Court's view that inferences can amount
to unconstitutional searches. The illegality in that case was "the
monitoring of a beeper in a private residence" to obtain
information that "could not have [been] obtained by observation
from outside," id., at 714-715, rather than any thought processes
that flowed from such monitoring. 46 justified, that certain facts will not come to the attention of
the authorities"). The interest in concealing the heat escaping
from one's house pales in significance to "the chief evil against
which the wording of the Fourth Amendment is directed," the
"physical entry of the home," United States v. United
States Dist. Court for Eastern Dist. of Mich., 407 U. S. 297,
313 (1972), and it is hard to believe that it is an interest the
Framers sought to protect in our Constitution.

															Since what was involved in this case was nothing more than
drawing inferences from off-the-wall surveillance, rather than any
"through-the-wall" surveillance, the officers' conduct did not
amount to a search and was perfectly reasonable.4

															II

															Instead of trying to answer the question whether the use of the
thermal imager in this case was even arguably unreasonable, the
Court has fashioned a rule that is intended to provide essential
guidance for the day when "more sophisticated systems" gain the
"ability to 'see' through walls and other opaque barriers." Ante, at 36, and n. 3. The newly minted rule encompasses
"obtaining [1] by senseenhancing technology [2] any information
regarding the interior of the home [3] that could not otherwise
have been obtained without physical intrusion into a
constitutionally protected area ... [4] at least where (as here)
the technology in question is not in general public use." Ante, at 34 (internal quotation marks omitted). In my
judgment, the

															4 This view comports with that of all the Courts of Appeals that
have resolved the issue. See 190 F.3d
1041 (CA9 1999); United States v. Robinson, 62 F.3d
1325 (CAll 1995) (upholding warrantless use of thermal imager); United States v. Myers, 46 F.3d
668 (CA7 1995) (same); United States v. Ishmael, 48 F.3d
850 (CA5 1995) (same); United States v. Pinson, 24 F.3d
1056 (CA8 1994) (same). But see United States v. Cusumano, 67 F.3d
1497 (CAW 1995) (warrantless use of thermal imager violated
Fourth Amendment), vacated and decided on other grounds, 83 F.3d
1247 (CAW 1996) (en bane). 47 Court's new rule is at once too broad and too narrow, and is not
justified by the Court's explanation for its adoption. As I have
suggested, I would not erect a constitutional impediment to the use
of sense-enhancing technology unless it provides its user with the
functional equivalent of actual presence in the area being
searched.

															Despite the Court's attempt to draw a line that is "not only
firm but also bright," ante, at 40, the contours of its new
rule are uncertain because its protection apparently dissipates as
soon as the relevant technology is "in general public use," ante, at 34. Yet how much use is general public use is not
even hinted at by the Court's opinion, which makes the somewhat
doubtful assumption that the thermal imager used in this case does
not satisfy that criterion.5 In any event, putting aside its lack
of clarity, this criterion is somewhat perverse because it seems
likely that the threat to privacy will grow, rather than recede, as
the use of intrusive equipment becomes more readily available.

															It is clear, however, that the category of "sense-enhancing
technology" covered by the new rule, ibid., is far too
broad. It would, for example, embrace potential mechanical
substitutes for dogs trained to react when they sniff narcotics.
But in United States v. Place, 462 U. S. 696 , 707
(1983), we held that a dog sniff that "discloses only the presence
or absence of narcotics" does "not constitute a 'search' within the
meaning of the Fourth Amendment," and it must follow that
sense-enhancing equipment that identifies nothing but illegal

															5 The record describes a device that numbers close to a thousand
manufactured units; that has a predecessor numbering in the
neighborhood of 4,000 to 5,000 units; that competes with a similar
product numbering from 5,000 to 6,000 units; and that is "readily
available to the public" for commercial, personal, or law
enforcement purposes, and is just an 800number away from being
rented from "half a dozen national companies" by anyone who wants
one. App. 18. Since, by virtue of the Court's new rule, the issue
is one of first impression, perhaps it should order an evidentiary
hearing to determine whether these facts suffice to establish
"general public use." 48 activity is not a search either. Nevertheless, the use of such a
device would be unconstitutional under the Court's rule, as would
the use of other new devices that might detect the odor of deadly
bacteria or chemicals for making a new type of high explosive, even
if the devices (like the dog sniffs) are "so limited both in the
manner in which" they obtain information and "in the content of the
information" they reveal. Ibid. If nothing more than that
sort of information could be obtained by using the devices in a
public place to monitor emissions from a house, then their use
would be no more objectionable than the use of the thermal imager
in this case.

															The application of the Court's new rule to "any information
regarding the interior of the home," ante, at 34, is also
unnecessarily broad. If it takes sensitive equipment to detect an
odor that identifies criminal conduct and nothing else, the fact
that the odor emanates from the interior of a home should not
provide it with constitutional protection. See supra, at 47
and this page. The criterion, moreover, is too sweeping in that
information "regarding" the interior of a home apparently is not
just information obtained through its walls, but also information
concerning the outside of the building that could lead to (however
many) inferences "regarding" what might be inside. Under that
expansive view, I suppose, an officer using an infrared camera to
observe a man silently entering the side door of a house at night
carrying a pizza might conclude that its interior is now occupied
by someone who likes pizza, and by doing so the officer would be
guilty of conducting an unconstitutional "search" of the home.

															Because the new rule applies to information regarding the
"interior" of the home, it is too narrow as well as too broad.
Clearly, a rule that is designed to protect individuals from the
overly intrusive use of sense-enhancing equipment should not be
limited to a home. If such equipment 49 did provide its user with the functional equivalent of access to
a private place-such as, for example, the telephone booth involved
in Katz, or an office building-then the rule should apply to
such an area as well as to a home. See Katz, 389 U. S., at
351 ("[T]he Fourth Amendment protects people, not places").

															The final requirement of the Court's new rule, that the
information "could not otherwise have been obtained without
physical intrusion into a constitutionally protected area," ante, at 34 (internal quotation marks omitted), also extends
too far as the Court applies it. As noted, the Court effectively
treats the mental process of analyzing data obtained from external
sources as the equivalent of a physical intrusion into the home.
See supra, at 44. As I have explained, however, the process
of drawing inferences from data in the public domain should not be
characterized as a search.

															The two reasons advanced by the Court as justifications for the
adoption of its new rule are both unpersuasive. First, the Court
suggests that its rule is compelled by our holding in Katz, because in that case, as in this, the surveillance consisted of
nothing more than the monitoring of waves emanating from a private
area into the public domain. See ante, at 35. Yet there are
critical differences between the cases. In Katz, the
electronic listening device attached to the outside of the phone
booth allowed the officers to pick up the content of the
conversation inside the booth, making them the functional
equivalent of intruders because they gathered information that was
otherwise available only to someone inside the private area; it
would be as if, in this case, the thermal imager presented a view
of the heat-generating activity inside petitioner's home. By
contrast, the thermal imager here disclosed only the relative
amounts of heat radiating from the house; it would be as if, in Katz, the listening device disclosed only the rela- 50 tive volume of sound leaving the booth, which presumably was
discernible in the public domain.6 Surely, there is a significant
difference between the general and well-settled expectation that
strangers will not have direct access to the contents of private
communications, on the one hand, and the rather theoretical
expectation that an occasional homeowner would even care if anybody
noticed the relative amounts of heat emanating from the walls of
his house, on the other. It is pure hyperbole for the Court to
suggest that refusing to extend the holding of Katz to this
case would leave the homeowner at the mercy of "technology that
could discern all human activity in the home." Ante, at
35-36.

															Second, the Court argues that the permissibility of
"through-the-wall surveillance" cannot depend on a distinction
between observing "intimate details" such as "the lady of the house
[taking] her daily sauna and bath," and noticing only "the
nonintimate rug on the vestibule floor" or "objects no smaller than
36 by 36 inches." Ante, at 37, 3839. This entire argument
assumes, of course, that the thermal imager in this case could or
did perform "through-thewall surveillance" that could identify any
detail "that would previously have been unknowable without physical
intrusion." Ante, at 39-40. In fact, the device could not,
see n. 1, supra, and did not, see Appendix, infra, enable its user to identify either the lady of the house, the rug
on the vestibule floor, or anything else inside the house, whether
smaller or larger than 36 by 36 inches. Indeed, the vague thermal
images of petitioner's home that are reproduced in the Appendix
were submitted by him to the District Court as part of an expert
report raising the question whether the device could even take
"accurate, consistent infrared images" of the

															6 The use of the latter device would be constitutional given
Smith v. Maryland, 442 U. S. 735 , 741
(1979), which upheld the use of pen registers to record numbers
dialed on a phone because, unlike "the listening device employed in Katz ... pen registers do not acquire the contents of
communications." 51 outside of his house. Defendant's Exh. 107, p. 4. But
even if the device could reliably show extraordinary differences in
the amounts of heat leaving his home, drawing the inference that
there was something suspicious occurring inside the residence-a
conclusion that officers far less gifted than Sherlock Holmes would
readily draw-does not qualify as "through-the-wall surveillance,"
much less a Fourth Amendment violation.

															III

															Although the Court is properly and commendably concerned about
the threats to privacy that may flow from advances in the
technology available to the law enforcement profession, it has
unfortunately failed to heed the tried and true counsel of judicial
restraint. Instead of concentrating on the rather mundane issue
that is actually presented by the case before it, the Court has
endeavored to craft an all-encompassing rule for the future. It
would be far wiser to give legislators an unimpeded opportunity to
grapple with these emerging issues rather than to shackle them with
prematurely devised constitutional constraints.

															I respectfully dissent.

															[Appendix to opinion of STEVENS, J., follows this page.] 52 Appendix to opinion of STEVENS, J.

															APPENDIX TO OPINION OF STEVENS, J. (Images and text reproduced
from defendant's exhibit 107) Top left: Infrared image of a video
frame from the videotape submitted as evidence in this case. The
thermogram indicates the suspect house as it appeared with the Gain
and contrast in its default setting. Only the outline of the house
is visible. The camera used was the Thermovision 210. Top Right:
Infrared image of a subsequent videoframe taken from the videotape.
The gain and contrast settings have been increased in order to make
the walls and roof of the structure appear hotter than what it
actually is. Bottom Left: Infrared image of the opposite side of the suspects
house. The thermogram is also taken from the same videotape. The
camera settings are in the default mode and the outline of the
house is barely visible. Only the hot electrical transformer and
the street light are identifiable.

															Bottom Right: The same image, but with the gain and contrast
increased. This change in camera settings cause any object to
appear hotter than what it actually is. The arrow indicates the
overloading of an area immediately around a hot object in this case
the electrical transformer and the streetlight. This overloading of
the image is an inherent design flaw in the camera itself.

Summary: The Supreme Court ruled that the use of a thermal-imaging device to scan a person's home without a warrant is a violation of the Fourth Amendment. The Court held that the government's use of a device not in general public use to explore the details of a private home constitutes a "search" and is presumptively unreasonable without a warrant. The case involved the use of a thermal-imaging device to detect marijuana growth in a home, and the Court ruled that the evidence obtained without a warrant should have been suppressed.


OPINION OF THE COURT UNITED STATES V. BANKS 540 U. S. ____ (2003) SUPREME COURT OF THE UNITED STATES NO. 02-473 UNITED STATES, PETITIONER v. LASHAWN
LOWELL BANKS

															on writ of certiorari to the united states court of
appeals for the ninth circuit

															[December 2, 2003]

															   Justice Souter delivered the
opinion of the Court.

															   Officers executing a warrant to
search for cocaine in respondent Banks’s apartment knocked and
announced their authority. The question is whether their
15-to-20-second wait before a forcible entry satisfied the Fourth
Amendment and 18 U. S. C. §3109. We hold that it did.

															I

															   With information that Banks was
selling cocaine at home, North Las Vegas Police Department officers
and Federal Bureau of Investigation agents got a warrant to search
his two-bedroom apartment. As soon as they arrived there, about 2
o’clock on a Wednesday afternoon, officers posted in front called
out “police search warrant” and rapped hard enough on the door to
be heard by officers at the back door. Brief for United States 3
(internal quotation marks omitted). There was no indication whether
anyone was home, and after waiting for 15 to 20 seconds with no
answer, the officers broke open the front door with a battering
ram. Banks was in the shower and testified that he heard nothing
until the crash of the door, which brought him out dripping to
confront the police. The search produced weapons, crack cocaine,
and other evidence of drug dealing.

															   In response to drug and firearms
charges, Banks moved to suppress evidence, arguing that the
officers executing the search warrant waited an unreasonably short
time before forcing entry, and so violated both the Fourth
Amendment and 18 U. S. C. §3109.[ Footnote 1 ] The District Court denied the motion, and
Banks pleaded guilty, reserving his right to challenge the search
on appeal.

															   A divided panel of the Ninth Circuit reversed
and ordered suppression of the evidence found. 282 F. 3d 699
(CA9 2002). In assessing the reasonableness of the execution of the
warrant, the panel majority set out a nonexhaustive list of
“factors that an officer reasonably should consider” in deciding
when to enter premises identified in a warrant, after knocking and
announcing their presence but receiving no express
acknowledgment:

															“(a) size of the residence; (b) location of the
residence; (c) location of the officers in relation to the main
living or sleeping areas of the residence; (d) time of day; (e)
nature of the suspected offense; (f) evidence demonstrating the
suspect’s guilt; (g) suspect’s prior convictions and, if any, the
type of offense for which he was convicted; and (h) any other
observations triggering the senses of the officers that reasonably
would lead one to believe that immediate entry was necessary.” Id. , at 704.

															   The majority also defined four
categories of intrusion after knock and announcement, saying that
the classification “aids in the resolution of the essential
question whether the entry made herein was reasonable under the
circumstances”:

															“(1) entries in which exigent circumstances exist
and non-forcible entry is possible, permitting entry to be made
simultaneously with or shortly after announcement; (2) entries in
which exigent circumstances exist and forced entry by destruction
of property is required, necessitating more specific inferences of
exigency; (3) entries in which no exigent circumstances exist and
non-forcible entry is possible, requiring an explicit refusal of
admittance or a lapse of a significant amount of time; and (4)
entries in which no exigent circumstances exist and forced entry by
destruction of property is required, mandating an explicit refusal
of admittance or a lapse of an even more substantial amount of
time.” Ibid. The panel majority put the action of the
officers here in the last category, on the understanding that they
destroyed the door without hearing anything to suggest a refusal to
admit even though sound traveled easily through the small
apartment. The majority held the 15-to-20-second delay after
knocking and announcing to be “[in]sufficient … to satisfy the
constitutional safeguards.” Id. , at 705.

															   Judge Fisher dissented, saying that the
majority ought to come out the other way based on the very grounds
it stressed: Banks’s small apartment, the loud knock and
announcement, the suspected offense of dealing in cocaine, and the
time of the day. Judge Fisher thought the lapse of 15 to 20 seconds
was enough to support a reasonable inference that admittance had
been constructively denied. Id. , at 710. We granted certiorari to consider how
to go about applying the standard of reasonableness to the length
of time police with a warrant must wait before entering without
permission after knocking and announcing their intent in a felony
case. 537 U. S.
1187 (2003). We now reverse.

															II

															   There has never been a dispute
that these officers were obliged to knock and announce their
intentions when executing the search warrant, an obligation they
concededly honored. Despite this agreement, we start with a word
about standards for requiring or dispensing with a knock and
announcement, since the same criteria bear on when the officers
could legitimately enter after knocking.

															   The Fourth Amendment says nothing
specific about formalities in exercising a warrant’s authorization,
speaking to the manner of searching as well as to the legitimacy of
searching at all simply in terms of the right to be “secure …
against unreasonable searches and seizures.” Although the notion of
reasonable execution must therefore be fleshed out, we have done
that case by case, largely avoiding categories and protocols for
searches. Instead, we have treated reasonableness as a function of
the facts of cases so various that no template is likely to produce
sounder results than examining the totality of circumstances in a
given case; it is too hard to invent categories without giving
short shrift to details that turn out to be important in a given
instance, and without inflating marginal ones. See, e.g ., Ohio v. Robinette , 519 U. S. 33 , 39 (1996)
(“[W]e have consistently eschewed bright-line rules, instead
emphasizing the fact-specific nature of the reasonableness
inquiry”); Ker v. California , 374 U. S. 23 , 33 (1963)
(reasonableness not susceptible to Procrustean application); Go-Bart Importing Co. v. United States , 282 U. S. 344 , 357
(1931) (no formula for determining reasonableness; each case on its
own facts and circumstances). We have, however, pointed out factual
considerations of unusual, albeit not dispositive,
significance.

															   In Wilson v. Arkansas , 514 U. S. 927 (1995), we held that the common law knock-and-announce principle is
one focus of the reasonableness enquiry; and we subsequently
decided that although the standard generally requires the police to
announce their intent to search before entering closed premises,
the obligation gives way when officers “have a reasonable suspicion
that knocking and announcing their presence, under the particular
circumstances, would be dangerous or futile, or … would inhibit the
effective investigation of the crime by, for example, allowing the
destruction of evidence,” Richards v. Wisconsin , 520 U. S. 385 , 394 (1997). When a
warrant applicant gives reasonable grounds to expect futility or to
suspect that one or another such exigency already exists or will
arise instantly upon knocking, a magistrate judge is acting within
the Constitution to authorize a “no-knock” entry.[ Footnote 2 ] And even when executing a
warrant silent about that, if circumstances support a reasonable
suspicion of exigency when the officers arrive at the door, they
may go straight in. Id. , at 394, 396, n. 7.

															   Since most people keep their doors locked,
entering without knocking will normally do some damage, a
circumstance too common to require a heightened justification when
a reasonable suspicion of exigency already justifies an unwarned
entry. We have accordingly held that police in exigent
circumstances may damage premises so far as necessary for a
no-knock entrance without demonstrating the suspected risk in any
more detail than the law demands for an unannounced intrusion
simply by lifting the latch. United States v. Ramirez , 523
U. S. 65 , 70–71 (1998). Either way, it is enough that the
officers had a reasonable suspicion of exigent
circumstances.[ Footnote 3 ]

															III

															   Like Ramirez , this case
turns on the significance of exigency revealed by circumstances
known to the officers, for the only substantive difference between
the two situations goes to the time at which the officers
reasonably anticipated some danger calling for action without
delay.[ Footnote 4 ] Whereas the Ramirez Magistrate Judge found in advance that the
customary warning would raise an immediate risk that a wanted felon
would elude capture or pose a threat to the officers, see id. , at 68, here the Government claims that a risk of
losing evidence arose shortly after knocking and announcing.
Although the police concededly arrived at Banks’s door without
reasonable suspicion of facts justifying a no-knock entry, they
argue that announcing their presence started the clock running
toward the moment of apprehension that Banks would flush away the
easily disposable cocaine, prompted by knowing the police would
soon be coming in. While it was held reasonable for the police in Ramirez to enter forcibly upon arrival, the Government
argues it was equally reasonable for the officers to go in with
force here as soon as the danger of disposal had ripened.

															   Banks does not, of course, deny
that exigency may develop in the period beginning when officers
with a warrant knock to be admitted, and the issue comes down to
whether it was reasonable to suspect imminent loss of evidence
after the 15 to 20 seconds the officers waited prior to forcing
their way. Though we agree with Judge Fisher’s dissenting opinion
that this call is a close one, 282 F. 3d, at 707, we think
that after 15 or 20 seconds without a response, police could fairly
suspect that cocaine would be gone if they were reticent any
longer. Courts of Appeals have, indeed, routinely held similar wait
times to be reasonable in drug cases with similar facts including
easily disposable evidence (and some courts have found even shorter
ones to be reasonable enough).[ Footnote 5 ]

															   A look at Banks’s counterarguments shows why
these courts reached sensible results, for each of his reasons for
saying that 15 to 20 seconds was too brief rests on a mistake about
the relevant enquiry: the fact that he was actually in the shower
and did not hear the officers is not to the point, and the same is
true of the claim that it might have taken him longer than 20
seconds if he had heard the knock and headed straight for the door.
As for the shower, it is enough to say that the facts known to the
police are what count in judging reasonable waiting time, cf., e.g. , Graham v. Connor , 490 U. S. 386 , 396
(1989) (“The ‘reasonableness’ of a particular use of force must be
judged from the perspective of a reasonable officer on the scene,
rather than with the 20/20 vision of hindsight.”), and there is no
indication that the police knew that Banks was in the shower and
thus unaware of an impending search that he would otherwise have
tried to frustrate.

															   And the argument that 15 to 20 seconds was too
short for Banks to have come to the door ignores the very risk that
justified prompt entry. True, if the officers were to justify their
timing here by claiming that Banks’s failure to admit them fairly
suggested a refusal to let them in, Banks could at least argue that
no such suspicion can arise until an occupant has had time to get
to the door,[ Footnote 6 ] a time
that will vary with the size of the establishment, perhaps five
seconds to open a motel room door, or several minutes to move
through a townhouse. In this case, however, the police claim
exigent need to enter, and the crucial fact in examining their
actions is not time to reach the door but the particular exigency
claimed. On the record here, what matters is the opportunity to get
rid of cocaine, which a prudent dealer will keep near a commode or
kitchen sink. The significant circumstances include the arrival of
the police during the day, when anyone inside would probably have
been up and around, and the sufficiency of 15 to 20 seconds for
getting to the bathroom or the kitchen to start flushing cocaine
down the drain. That is, when circumstances are exigent because a
pusher may be near the point of putting his drugs beyond reach, it
is imminent disposal, not travel time to the entrance, that governs
when the police may reasonably enter; since the bathroom and
kitchen are usually in the interior of a dwelling, not the front
hall, there is no reason generally to peg the travel time to the
location of the door, and no reliable basis for giving the
proprietor of a mansion a longer wait than the resident of a
bungalow, or an apartment like Banks’s. And 15 to 20 seconds does
not seem an unrealistic guess about the time someone would need to
get in a position to rid his quarters of cocaine.

															   Once the exigency had matured, of course, the
officers were not bound to learn anything more or wait any longer
before going in, even though their entry entailed some harm to the
building. Ramirez held that the exigent need of law
enforcement trumps a resident’s interest in avoiding all property
damage, see 523 U. S., at 70–71, and there is no reason to treat a
post-knock exigency differently from the no-knock counterpart in Ramirez itself.

															IV

															   Our emphasis on totality analysis
necessarily rejects positions taken on each side of this case. Ramirez , for example, cannot be read with the breadth the
Government espouses, as “reflect[ing] a general principle that the
need to damage property in order to effectuate an entry to execute
a search warrant should not be part of the analysis of whether the
entry itself was reasonable.” Brief for the United States 18; Reply
Brief for United States 4. At common law, the knock-and-announce
rule was traditionally “justified in part by the belief that
announcement generally would avoid ‘the destruction or breaking of
any house … by which great damage and inconvenience might
ensue.’ ” Wilson , 514 U. S., at 935–936 (quoting Semayne’s Case , 5 Co. Rep. 91a, 91b, 77 Eng. Rep. 194, 196
(K. B. 1603)). One point in making an officer knock and announce,
then, is to give a person inside the chance to save his door. That
is why, in the case with no reason to suspect an immediate risk of
frustration or futility in waiting at all, the reasonable wait time
may well be longer when police make a forced entry, since they
ought to be more certain the occupant has had time to answer the
door. It is hard to be more definite than that, without turning the
notion of a reasonable time under all the circumstances into a set
of sub-rules as the Ninth Circuit has been inclined to do. Suffice
it to say that the need to damage property in the course of getting
in is a good reason to require more patience than it would be
reasonable to expect if the door were open. Police seeking a stolen
piano may be able to spend more time to make sure they really need
the battering ram.

															   On the other side, we disapprove
of the Court of Appeals’s four-part scheme for vetting
knock-and-announce entries. To begin with, the demand for enhanced
evidence of exigency before a door can reasonably be damaged by a
warranted no-knock intrusion was already bad law before the Court
of Appeals decided this case. In Ramirez (a case from the
Ninth Circuit), we rejected an attempt to subdivide felony cases by
accepting “mild exigency” for entry without property damage, but
requiring “more specific inferences of exigency” before damage
would be reasonable. 523 U. S., at 69–71 (internal quotation marks
omitted). The Court of Appeals did not cite Ramirez .

															   Nor did the appeals court cite United
States v. Arvizu , 534 U. S. 266 (2002)
(again, from the Ninth Circuit). There, we recently disapproved a
framework for making reasonable suspicion determinations that
attempted to reduce what the Circuit described as “troubling …
uncertainty” in reasonableness analysis, by “describ[ing] and
clearly delimit[ing]” an officer’s consideration of certain
factors. Id. , at 272, 275 (internal quotation marks
omitted). Here, as in Arvizu , the Court of Appeals’s
overlay of a categorical scheme on the general reasonableness
analysis threatens to distort the “totality of the circumstances”
principle, by replacing a stress on revealing facts with resort to
pigeonholes. Id. , at 274 (internal quotation marks
omitted). Attention to cocaine rocks and pianos tells a lot about
the chances of their respective disposal and its bearing on
reasonable time. Instructions couched in terms like “significant
amount of time,” and “an even more substantial amount of time,” 282
F. 3d, at 704, tell very little.

															V

															   Last, there is Banks’s claim that
the entry violated 18 U. S. C. §3109. Ramirez held
that the result should be the same under the Fourth Amendment and
§3109, permitting an officer to enter by force “if, after notice of
his authority and purpose, he is refused admittance.” We explained
the statute’s “ ‘requirement of prior notice … before forcing
entry … [as] codif[ying] a tradition embedded in Anglo-American
law,’ ” 523 U. S., at 72 (quoting Miller v. United States , 357 U. S. 301 , 313
(1958)); see also Sabbath v. United States , 391 U. S. 585 ,
591, n. 8 (1968), and we held that §3109 implicates the
exceptions to the common law knock-and-announce requirement that
inform the Fourth Amendment itself, 523 U. S., at 73. The upshot is
that §3109 is subject to an exigent circumstances exception, ibid. , which qualifies the requirement of refusal after
notice, just as it qualifies the obligation to announce in the
first place. Absent exigency, the police must knock and receive an
actual refusal or wait out the time necessary to infer one. But in
a case like this, where the officers knocked and announced their
presence, and forcibly entered after a reasonable suspicion of
exigency had ripened, their entry satisfied §3109 as well as the
Fourth Amendment, even without refusal of admittance.

															   The judgment of the Court of
Appeals is reversed.

															So ordered. Footnote 1 The statute provides: “The officer may break
open any outer or inner door or window of a house, or any part of a
house, or anything therein, to execute a search warrant, if, after
notice of his authority and purpose, he is refused admittance or
when necessary to liberate himself or a person aiding him in the
execution of the warrant.” Footnote 2 Some States give magistrate judges the
authority to issue “no-knock” warrants, and some do not. See, e.g., Richards v. Wisconsin , 520 U. S. 385 , 396, n. 7 (1997)
(collecting state statutes and cases). Footnote 3 The standard for a no-knock entry stated in Richards applies on reasonable suspicion of exigency or
futility. Because the facts here go to exigency, not futility, we
speak of that alone. Footnote 4 Ramirez and Richards , our
cases addressing the role of exigency in assessing the
reasonableness of a no-knock entry, involved searches by warrant
for evidence of a felony, as does this case. In a different context
governed by the Fourth Amendment, we have held that the risk of
losing evidence of a minor offense is insufficient to make it
reasonable to enter a dwelling to make a warrantless arrest. See Welsh v. Wisconsin , 466 U. S. 740 (1984).
Courts of Appeals have applied Welsh to warrantless
entries simply to search for evidence, considering the gravity of
the offense in determining whether exigent circumstances exist.
See , e.g. , United States v. Aquino , 836
F. 2d 1268, 1271–1273 (CA10 1988); United States v. Clement , 854 F. 2d 1116, 1120 (CA8 1988). We intimate
nothing here about such warrantless entry cases. Nor do we express
a view on the significance of the existence of a warrant in
evaluating whether exigency justifies action in knock-and-announce
cases when the reason for the search is a minor offense. Footnote 5 Several Courts of Appeals have explicitly
taken into account the risk of disposal of drug evidence as a
factor in evaluating the reasonableness of waiting time. See, e.g. , United States v. Goodson , 165
F. 3d 610, 612, 614 (CA8 1999) (holding a 20–second wait after
a loud announcement at a one-story ranch reasonable); United
States v. Spikes , 158 F. 3d 913, 925–927 (CA6
1998) (holding a 15-to-30-second wait in midmorning after a loud
announcement reasonable); United States v. Spriggs , 996 F. 2d 320, 322–323 (CADC 1993) (holding
a 15-second wait after a reasonably audible announcement at 7:45
a.m. on a weekday reasonable); United States v. Garcia , 983 F. 2d 1160, 1168 (CA1 1993) (holding a
10-second wait after a loud announcement reasonable); United
States v. Jones , 133 F. 3d 358, 361–362 (CA5
1998) (relying specifically on the concept of exigency, holding a
15-to-20-second wait reasonable). See also United States v. Chavez-Miranda , 306 F. 3d 973, 981–982, n. 7
(CA9 2002) (“ Banks appears to be a departure from our
prior decisions… .[W]e have found a 10 to 20 second wait to be
reasonable in similar circumstances, albeit when the police heard
sounds after the knock and announcement”); United States v. Jenkins , 175 F. 3d 1208, 1215 (CA10 1999) (holding
a 14-to-20-second wait at 10 a.m. reasonable); United
States v. Markling , 7 F. 3d 1309, 1318–1319 (CA7
1993) (holding a 7-second wait at a small motel room reasonable
when officers acted on a specific tip that the suspect was likely
to dispose of the drugs). Footnote 6 It is probably unrealistic even on its own
terms. The apartment was “small,” 282 F. 3d 699, 704 (CA9
2002), and a man may walk the length of today’s small apartment in
15 seconds.

Summary: The Supreme Court held that a 15-20 second wait after knocking and announcing their presence before forcibly entering a residence to execute a search warrant was reasonable and did not violate the Fourth Amendment or 18 U.S.C. §3109. The Court considered factors such as the size and location of the residence, the time of day, and the nature of the suspected offense in determining that the entry was reasonable.