In the course of arresting Riley, police officers confiscated and searched his cell phone. On it they found evidence suggesting his involvement in a drive-by shooting.

Mr. Riley was charged with attempted murder.

Despite all that, the Supreme Court ruled that the police violated Riley’s Fourth Amendment rights by searching the cell phone in his pocket upon his arrest. Evidence of attempted murder was excluded for lack of a proper search warrant.

Why was a warrant needed, given that police officers are perfectly entitled to examine most physical objects that are found on an individual being lawfully arrested?

Roberts reasoned that unlike most physical objects, the digital contents of a cell phone neither threaten the physical safety of police nor risk aiding arrestees in an escape nor threaten the preservation of evidence. The state has a much lesser interest in conducting an immediate search. And when balancing state interests against the individual’s interest in privacy, cell phones are different in that they “place vast quantities of personal information literally in the hands of individuals.”

Roberts spent a lot of time emphasizing that final point.

“Before cell phones, a search of a person was limited by physical realities and tended as a general matter to constitute only a narrow intrusion on privacy. Most people cannot lug around every piece of mail they have received for the past several months, every picture they have taken, or every book or article they have read—nor would they have any reason to attempt to do so,” Roberts explained. “Now the person who is not carrying a cell phone, with all it contains, is the exception.”

He acknowledged his ruling would impose a burden on police:

We cannot deny that our decision today will have an impact on the ability of law enforcement to combat crime. Cell phones have become important tools in facilitating coordination and communication among members of criminal enterprises, and can provide valuable incriminating information about dangerous criminals.

But he reasoned that even a person’s home, where Americans traditionally enjoyed the highest degree of Fourth Amendment protection, might contain but a fraction of the private details a cell phone would––and that protecting privacy has costs.

“Cell phones are not just another convenience,” Roberts wrote. “With all they contain and all they may reveal, they hold for many Americans ‘the privacies of life.’ That technology now allows an individual to carry such information in his hand does not make the information any less worthy of the protection for which the Founders fought. Our answer to the question of what police must do before searching a cell phone seized incident to an arrest is accordingly simple— get a warrant.”

Ever since, a warrant has been required to search the cell phones of people who are being arrested, even for the most serious crimes. And that standard raises a question: