chief justice john roberts delivered the opinion of the court, concluding that a warrant is required to search a mobile phone. roberts wrote that it fails the warrantless search test established in "chimel v. california": digital data stored on a cell phone cannot itself be used as a weapon to harm an arresting officer or to effectuate the arrestee's escape. law enforcement officers remain free to examine the physical aspects of a phone to ensure that it will not be used as a weapon--say, to determine whether there is a razor blade hidden between the phone and its case. once an officer has secured a phone and eliminated any potential physical threats, however, data on the phone can endanger no one.