Updated at 12:50 p.m. ET 
 
 Paul Kevin Curtis, 45 was arrested at his home in Corinth, Tenn., April 17, 2013. / AP 
 
 A U.S. official has confirmed an arrest in the investigation of recent mailings of suspicious letters to President Obama and a senator, CBS News correspondent Bob Orr reported Wednesday evening. 
 
 In a statement, the Department of Justice announced the arrest of Paul Kevin Curtis, "the individual believed to be responsible for the mailings of the three letters sent through the U.S. Postal Service which contained a granular substance that preliminarily tested positive for ricin. The letters were addressed to a U.S. Senator, the White House and a Mississippi justice official." It added that Curtis was arrested at his home in Corinth, Miss. 
 
 When the test results on one of the letters, addressed to Roger Wicker, R-Miss., came back from the Ft. Dietrick lab Wednesday, various officials described them as "weak," "low-grade" and "less than one percent toxin," CBS News has learned. 
 
 A Prince George's County, Md. firefighter dressed in a protective suit walks out of a government mail screening facility in Hyattsville, Md., Wednesday, April 17, 2013. / AP Photo/Alex Brandon 
 
 Threatening letters to the president and congressional leaders are not unusual -- their mail is processed at a facility about 10 miles outside of Washington in Landover, Md. The current security protocols -- which seem to be working exactly as they were set to do -- were put in place after the 2001 anthrax scare that targeted congressional leaders. 
 
 In addition to the letter sent to Obama this week, two letters being tested for ricin were sent to Sen. Roger Wicker, R-Miss. At least one of the letters addressed to Wicker tested positive in a preliminary field test for ricin. 
 
 The envelope addressed to the White House, the FBI said in a statement Wednesday, also initially tested positive for ricin and was immediately quarantined by U.S. Secret Service personnel. A coordinated investigation with the FBI was initiated. Field tests are regularly inaccurate; therefore, the FBI is conducting more thorough tests on the letters. 
 
 At a press briefing Wednesday, White House spokesman Jay Carney referred questions about the letters to the FBI, adding, "The president has of course been briefed on these letters. He was briefed last night and again this morning." 
 
 The letters came from the Memphis area, and were sent the same day, April 8. The letters, CBS News has confirmed, said in part, "No one wanted to listen to me before. This must stop. To see a wrong and not expose it is to become a silent partner to its continuance. I am KC and I approve this message." 
 
 After the arrest was announced, a Mississippi state lawmaker, Democratic Rep. Steve Holland of Plantersville, said his 80-year-old mother, Lee County Justice Court Judge Sadie Holland, received a threatening letter last week with a substance that has been sent to a lab for testing. He said this letter was also signed "K.C.". 
 
 As authorities scurried to investigate three questionable packages discovered in Senate office buildings, reports of suspicious items also came in from at least three senators' offices in their home states. 
 
 Sen. Carl Levin said a staff member at his Saginaw, Mich., office would spend the night in a hospital as a precaution after discovering a suspicious letter. The staff member had no symptoms, Levin said in a statement. He expected to learn preliminary results of tests on the letter by Thursday. 
 
 Sen. Jeff Flake, R-Ariz., said suspicious letters at his Phoenix office had been cleared with nothing dangerous found. A package at Sen. John Cornyn's Dallas-area office also was declared harmless, a fire department spokesman said. 
 
 All three packages in the Capitol complex turned out to be safe, Capitol police spokeswoman Makema Turner said late Wednesday. But a man was still being questioned after being stopped in connection with the packages, she said. 
 
 All the activity came as tensions were high in Washington and across the country following Monday's bombings at the Boston Marathon that killed three people and injured more than 170. 
 
 At a senators-only briefing on Tuesday about the bombing, senators were also briefed by Sergeant at Arms Terence Gainer about the Wicker letter that tested positive in initial tests. Sen. Claire McCaskill, D-Mo., told reporters that the senators were told the suspected letter writer writes a lot of letters to members. 
 
 "The investigation into these letters remains ongoing, and more letters may still be received," the FBI said in its statement. "There is no indication of a connection to the attack in Boston." 

 The ricin-laced letters sent to President Obama and Mississippi Sen. Roger Wicker were both postmarked April 8 and sent from Memphis, Tenn., signed "I am KC and I approve this message." A third letter went to Michigan Sen. Carl Levin. NBC's Andrea Mitchell reports. 
 
 Federal agents on Wednesday arrested a suspect in the mailing of letters to President Barack Obama and a U.S. senator that initially tested positive for the poison ricin. 
 
 The suspect was identified as Paul Kevin Curtis of Tupelo, Miss., federal officials told NBC News. They said he may appear in court as early as Wednesday night. 
 
 Both letters carried an identical closing statement, according to an FBI bulletin obtained by NBC News on Wednesday. 
 
 According to the FBI bulletin, both letters, postmarked April 8, 2013 out of Memphis, Tenn., included an identical phrase, "to see a wrong and not expose it, is to become a silent partner to its continuance." 
 
 In addition, both letters are signed: "I am KC and I approve this message." 
 
 The letter to Obama was intercepted at an off-site White House mail facility and was being tested further, the FBI said. A federal law enforcement official said that the letter was “very similar” to one addressed to Sen. Roger Wicker, R-Miss. Another letter was addressed to a Mississippi justice official. 
 
 Lee County Sheriff Jim Johnson confirms a suspect has been arrested in relation to the ricin-laced letter sent to a Mississippi judge. Johnson adds that there are "great consistencies" between this letter and those sent to President Obama and Senator Wicker. 
 
 Curtis was arrested at his home in Corinth, Miss., at about 5:15 CT, after an investigation by federal, state and local agencies. 
 
 In a news conference Wednesday night, Lee County, Miss., Sheriff Jim Johnson said a Mississippi judge on April 10 received and opened a typewritten letter -- postmarked from Memphis, but without a return address -- that included “wording that was of interest,” as well as some "suspicious content." 
 
 Johnson said there were “great consistencies” between the letter received by the Mississippi judge and the letters directed to Wicker and President Obama. Tests are being conducted on the letter sent to the Mississippi judge to determine whether it was tainted with ricin, Johnson added. 
 
 The FBI is assisting the sheriff’s office in the investigation to determine whether the letters were sent by the same person. 
 
 Johnson would not identify the suspect in custody, adding that local authorities are waiting for results from the federal laboratory before filing any state charges. 
 
 Two federal officials said late Wednesday that an initial laboratory test on the material in the letters was inconclusive. The test shows some level of ricin, they said, but the potency is uncertain. They cannot tell whether the material is actually harmful or not, so more tests have been ordered. 
 
 Wicker released a statement Wednesday, thanking authorities for their help. 
 
 "Gayle and I want to thank the men and women of the FBI and U.S. Capitol Police for their professionalism and decisive action in keeping our family and staff safe from harm," the statement read. 
 
 "My offices in Mississippi and Washington remain open for business to all Mississippians. We particularly want to thank the people of Mississippi for their thoughts and prayers during this time." 
 
 The sender of the letters, one official said, "may have stumbled onto something," but it's unknown if he actually made the full-blown ricin toxin. 
 
 Ricin is made from castor beans and can kill within 36 hours. There is no antidote. Some threatening letters simply contain ground castor beans, resulting in a positive field test for ricin without the concentrated poison. Results from full laboratory tests are expected in the next 24 to 48 hours. 
 
 Filters at a second government mail screening facility also tested positive for ricin in a preliminary screening Wednesday. 
 
 An FBI official told NBC News that the agency did not initially believe the letters were related to the attack on the Boston Marathon on Monday. 
 
 Authorities also for a time cleared the atrium of a Senate office building Wednesday, removing suspicious envelopes and a package, before reopening the offices. Capitol police were also investigating a suspicious package at the office of Sen. Richard Shelby, R-Ala. Shelby’s staff had not been evacuated. 
 
 The Wicker letter had no return address. The FBI confirmed the preliminary positive test on it Tuesday. That letter was intercepted at a postal facility in Maryland that screens mail sent to Congress, and never reached Wicker’s office. 
 
 Other senators were made aware of the Wicker letter during a briefing Tuesday evening on the bombing in Boston. Sen. Claire McCaskill, D-Mo., said that the person who sent Wicker the letter writes often to elected officials. 
 
 People can be exposed to ricin by touching a ricin-laced letter or by inhaling particles that enter the air when the envelope is opened. Touching ricin can cause a rash but is not usually fatal. Inhaling it can cause trouble breathing, fever and other symptoms, and can be fatal. 
 
 At a hearing Wednesday on the Postal Service’s finances, Postmaster General Patrick Donahoe said that while there have been ricin scares in the past, the recent discoveries were unprecedented. 
 
 “There's never been any actually proved that have gone through the system,” Donahoe said. “But we've got a process that we make sure that our employees know -- We can actually track the mail back through the system to double check from an employee health standpoint." 
 
 Field tests are conducted anytime suspicious powder is found in a mail facility, and the FBI cautioned that field tests and other preliminary tests can produce inconsistent results. When tests show the possibility of a biological agent, the material is sent to a laboratory for full analysis. 
 
 Robert Windrem, Kasie Hunt, Kelly O’Donnell, Richard Esposito, Jeff Black, Mike Viqueira and Dr. Kristina Krohn of NBC News contributed to this report. 
 
 Editor's note: An earlier version of this story misidentified the suspect, based on information from federal officials. 
 
 Related: 
 
 Deadly ricin: Poisonous but clumsy weapon 
 
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– Federal authorities have arrested a man identified as Paul Kevin Curtis of Tupelo, Mississippi, in the mailing of letters laced with toxin to President Obama and Sen. Roger Wicker, reports the Clarion-Ledger. Not much is known about the suspect at this point, though both letters were signed, "I am KC and I approve this message." Each also contained the message, "to see a wrong and not expose it, is to become a silent partner to its continuance." The letters were flagged at mail-sorting facilities before reaching their intended targets. Initial tests turned up positive for the poison ricin, but NBC News reports that it's still not clear just how potent the letters were. Further tests are under way. The letters were postmarked on April 8. CBS News says there were three in all, one to Wicker, one to Obama, and one to a Mississippi justice official. (Claire McCaskill had previously said that a suspect identified in the case was known to be a prolific letter-writer to lawmakers.)