The Supreme Court on Wednesday ruled unanimously that the authorities generally may not search the mobile phones of those they arrest unless they have a court warrant.

"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," Chief Justice John Roberts wrote (PDF). It's perhaps the biggest digital-age privacy decision that the high court has rendered following its 2012 ruling that the authorities usually need warrants to affix GPS trackers to a suspect's vehicle.

The Obama administration and prosecutors from states across the country had lobbied the high court in briefs to allow police officers to be able to search arrestees' gadgets—not just mobile phones—without a warrant. The justices declined to do so, saying that "privacy comes at a cost."

"Modern cell phones, as a category, implicate privacy concerns far beyond those implicated by the search of a cigarette pack, a wallet, or a purse. A conclusion that inspecting the contents of an arrestee’s pockets works no substantial additional intrusion on privacy beyond the arrest itself may make sense as applied to physical items, but any extension of that reasoning to digital data has to rest on its own bottom," the court ruled.

The court added: "Prior to the digital age, people did not typically carry a cache of sensitive personal information with them as they went about their day. Now it is the person who is not carrying a cellphone, with all that it contains, who is the exception."

The court suggested that it would not tolerate warrantless mobile phone tracking, either. The lower courts are mixed on whether the authorities need a warrant to track a suspect's every move.

... it is no exaggeration to say that many of the more than 90% of American adults who own a cell phone keep on their person a digital record of nearly every aspect of their lives—from the mundane to the intimate. See Ontario v. Quon, 560 U. S. 746, 760 (2010). Allowing the police to scrutinize such records on a routine basis is quite different from allowing them to search a personal item or two in the occasional case. Although the data stored on a cell phone is distinguished from physical records by quantity alone, certain types of data are also qualitatively different. An Internet search and browsing history, for example, can be found on an Internet-enabled phone and could reveal an individual’s private interests or concerns—perhaps a search for certain symptoms of disease, coupled with frequent visits to WebMD. Data on a cell phone can also reveal where a person has been. Historic location information is a standard feature on many smart phones and can reconstruct someone's specific movements down to the minute, not only around town but also within a particular building.

The justices, however, said that waiting for a judge to issue a warrant is not needed in every case if there are "exigent," or emergency, circumstances:

In light of the availability of the exigent circumstances exception, there is no reason to believe that law enforcement officers will not be able to address some of the more extreme hypotheticals that have been suggested: a suspect texting an accomplice who, it is feared, is preparing to detonate a bomb, or a child abductor who may have information about the child’s location on his cell phone. The defendants here recognize—indeed, they stress—that such fact-specific threats may justify a warrantless search of cell phone data.

The justices ruled on two cases.

One concerned an accused Boston crack dealer sentenced to 22 years in prison following a 2007 arrest. The police suspected Brima Wurie was selling drugs from his vehicle, so they arrested Wurie, confiscated his phone, and reviewed his calling logs. According to court records, a call from "my house" repeatedly kept appearing on the phone's external screen. Once the cops opened the phone, they saw a picture of a woman holding a baby.

Authorities traced the "my house" number and suspected that the address was where a "hidden mother cache" of crack cocaine might have been stashed. Further, the "my house" address was different from the one Wurie supplied to police when he was arrested.

Police went to the "my house" residence and discovered the mailbox displayed Wurie's name. A woman the police saw through the apartment's window matched the phone's wallpaper picture, court records show. With a search warrant, the cops found a firearm, ammunition, marijuana, and crack cocaine inside the residence. Wurie challenged the search.

On appeal before a federal appellate panel, the defendant successfully claimed that the phone search was a breach of the Fourth Amendment. The precedent first established by the Supreme Court in 1914 that allowed warrantless searches of arrestees did not apply to electronic devices, Wurie argued.

The First US Circuit Court of Appeals agreed and tossed evidence discovered in the search, resulting in two of three charges being dismissed. The appeals court ruled that "the search-incident-to-arrest exception does not authorize the warrantless search of data on a cell phone seized from an arrestee's person."

The government appealed to the Supreme Court. Among other things, Solicitor General Donald Verrilli Jr. argued that the justices "should not deprive officers of an investigative tool that is increasingly important for preserving evidence of serious crimes based on purely imaginary fears that police officers will invoke their authority to review drug dealers'... 'appointments with marital counselors' or armed robbers' 'apps to help smokers quit.'" (Verrilli was citing examples lodged with the court by the Electronic Frontier Foundation and Center for Democracy and Technology.)

The government also argued that even if the justices were to disavow wanton gadget searches, the high court should at least allow the authorities to review a phone's calling logs. The high court did not bite and upheld the appellate court's ruling on Wednesday.

The other search case the justices decided Wednesday concerns a San Diego man, David Riley, who was pulled over for driving with expired tags in 2009. The college student was also operating a vehicle with a suspended driver's license. Riley was arrested, and when officers searched the Lexus he was driving, they discovered firearms concealed under the hood. The authorities also searched his Samsung Instinct M800 smartphone twice without warrants, once on the scene and again at the precinct.

Police discovered a picture taken by the phone of Riley posing with a gang member and a red Oldsmobile they believed was involved in an unsolved drive-by shooting. A ballistics test of the weapons seized from the Lexus concluded that at least one of them was used in the earlier gang shooting, according to the record.

Riley's first trial ended in a hung jury. But Riley was convicted on a retrial, without eyewitnesses, of shooting at an occupied vehicle, attempted murder, and assault with a semiautomatic weapon.

Prosecutors showed jurors the photo they seized from Riley's mobile phone. He was sentenced to 15 years to life in prison and appealed to the Supreme Court, which reversed his conviction.