Correction: An earlier version of the story misspelled the last name of an American University law professor. He is Stephen Vladeck. This version has been corrected.

Kirsten Luna from Michigan uses her smartphone outside the U.S. Supreme Court, which ruled on June 25 that police must have a warrant before searching cellphones. (Win Mcnamee/Getty Images)

The Supreme Court unequivocally ruled Wednesday that privacy rights are not sacrificed to 21st-
century technology, saying unanimously that police generally must obtain a warrant before searching the cellphone of someone they arrest.

While the specific protection may not affect the average American, the court made a bold statement that the same concern about government prying that animated the nation’s birth applies to the abundance of digital information about an individual in the modern world.

Modern cellphones “hold for many Americans the privacies of life,” Chief Justice John G. Roberts Jr. wrote for a court united behind the opinion’s expansive language. “The fact 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.”

Roberts said that in most cases when police seize a cellphone from a suspect, the answer is simple: “Get a warrant.”

(Read the cellphone decision)

The ruling has no impact on
National Security Agency data-
collection programs revealed in the past year or law enforcement use of aggregated digital information. But lawyers involved in those issues said the emphatic declarations signaled the justices’ interest in the dangers of government overreach.

Stephen Vladeck, a law professor at American University, said the decision is more than simply a warning to government officials employing high-tech forms of government surveillance.

“This is a cruise missile across the bow of lawyers defending warrantless search programs,” Vladeck said. “This will at least scare the bejesus out of them.”

During oral arguments, the justices seemed divided over the issue. But they united behind soaring language from Roberts about privacy concerns in the digital era in which 90 percent of Americans carry cellphones containing sensitive information.

“The term ‘cell phone’ is itself misleading shorthand; many of these devices are in fact minicomputers that also happen to have the capacity to be used as a telephone,” Roberts wrote. “They could just as easily be called cameras, video players, rolodexes, calendars, tape recorders, libraries, diaries, albums, televisions, maps, or newspapers.”

The court is often criticized for being behind the times in considering technological advances. But Roberts’s opinion was filled with geeky facts — “the average smart phone user has installed 33 apps, which together can form a revealing montage of the user’s life” — and concerns about modern innovations such as cloud computing — “cell phone users often may not know whether particular information is stored on the device or in the cloud.”

Jeffrey Fisher, a Stanford law professor who argued on behalf of a defendant who said the search violated his constitutional right to be free of unreasonable ­searches, praised the ruling.

“The decision brings the Fourth Amendment into the digital age,” Fisher said. “The core of the decision is that digital information is different. It triggers privacy concerns far more profound than ordinary physical objects.”

Ellen Canale, a Justice Department spokeswoman, said the department will work with law enforcement to ensure that the court’s decision is implemented.

“Our commitment to vigorously enforcing the criminal laws and protecting the public while respecting the privacy interests protected by the Fourth Amendment is unwavering,” she said.

In general, warrants are required for ­searches, but the court’s precedents have said that a person’s privacy expectations diminish considerably after an arrest. Police may protect themselves and others by searching the arrestee for weapons or securing evidence that might be destroyed.

Roberts said he “cannot deny” that the decision will have an impact on the ability of law enforcement to combat crime. “Privacy comes at a cost,” he wrote.

But he said police can use their own technology to ensure that the information on cellphones that might contain critical evidence is not erased or lost. He also said there could be “case-specific”exceptions to the warrant rule.

The court in the past had approved searching many objects found on a suspect, Roberts noted, including a cigarette pack found to have contained drugs. But allowing them to search a cellphone is more akin to ransacking a person’s home, he said.

“Indeed, a cell phone search would typically expose to the government far more than the most exhaustive search of a house: A phone not only contains in digital form many sensitive records previously found in the home; it also contains a broad array of private information never found in a home in any form,” he said. For instance: “Past location information is a standard feature on many smartphones and can reconstruct someone’s specific movements down to the minute, not only around town but also within a particular building.”

He said technology also makes it easier for law enforcement to secure approval from a judge that a search is justified. Canale said the Justice Department would work on that.

“We will make use of whatever technology is available to preserve evidence on cellphones while seeking a warrant, and we will assist our agents in determining when exigent circumstances or another applicable exception to the warrant requirement will permit them to search the phone immediately without a warrant,” she said.

Justice Samuel A. Alito filed an opinion concurring in the judgment, despite reservations about what it might mean for law enforcement.

He also urged legislatures and Congress to get involved.

“Many forms of modern technology are making it easier and easier for both government and private entities to amass a wealth of information about the lives of ordinary Americans, and at the same time, many ordinary Americans are choosing to make public much information that was seldom revealed to outsiders just a few decades ago,” Alito wrote. “In light of these developments, it would be very unfortunate if privacy protection in the 21st century were left primarily to the federal courts using the blunt instrument of the Fourth Amendment.”

The court ruling came in the consideration of two cases in which lower courts arrived at different conclusions.

One involved Brima Wurie, who was picked up in Boston on suspicion of selling crack cocaine in 2007. While he was in police custody, his flip-style phone kept receiving calls from a number identified as “my house.”

Using the telephone number and a reverse directory, police located his address, obtained a warrant to search his home, and found crack, marijuana and a weapon.

In a 2-to-1 decision, a panel of the U.S. Court of Appeals for the 1st Circuit threw out the evidence against Wurie. The majority endorsed a rule that said warrantless cellphone data searches are “categorically unlawful,” given the “government’s failure to demonstrate that they are ever necessary to promote officer safety or prevent the destruction of evidence.”

A case from California went the other way.

David Leon Riley was pulled over in 2009 by a San Diego police officer for an expired car registration. Police quickly discovered that Riley’s driver’s license was suspended and later found guns under the car’s hood.

Police also examined his smartphone and found language that led them to believe Riley had gang connections. A photograph on the phone linked him to a car that police said had been used to flee a shooting.

Riley was indicted on murder and other charges, convicted, and sentenced to more than 15 years in prison.

A California court upheld the officers’ actions, and similar conflicting decisions have been recorded across the country.

The cases are United States v. Wurie and Riley v. California.

An earlier version of this story misspelled the name of American University professor Stephen Vladeck. It has been corrected.

Craig Timberg contributed to this report.