WASHINGTON — Thousands of times a year, the nation's police departments get phone company records allowing them to plot the movements of individual customers. On Wednesday, the U.S. Supreme Court will consider whether access to that data should require a search warrant issued by a judge.
The question comes at a time when cellphones are nearly ubiquitous: 95 percent of Americans now own one.
The case is a challenge brought by a Michigan man, Timothy Carpenter, who was convicted of robbing a string of Radio Shack and T-Mobile stores after FBI agents used three months of cellphone records to show that he was near each one at the time of the crimes.
He argues that because the FBI did not get a search warrant, that evidence, along with his conviction, should be thrown out.
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When a cellphone is used for calls or texts, it signals a nearby antenna tower to connect with the telephone network. As the user travels, the call is handed off to successive towers, and the cellphone companies keep records of the phone numbers routed through each tower to sort out such things as roaming charges.
Carpenter lost in the lower courts, which ruled that no search warrant was needed because phone customers have no expectation that their records will be private. They relied on a Supreme Court decision from 38 years ago, which said phone customers don't expect that the numbers they dial will remain private, because the phone company uses that information for billing.
But Carpenter's lawyers say the reasoning of that decades-old case, when telephones were hard-wired into a wall and didn't move, should not apply in the digital age. Getting data on cellphone tower locations, they argue, allows the police to discover far more than which numbers were dialed.
"Knowing where a person's phone goes can tell you a great deal of private information about them, from where someone slept at night — at home, or in someone else's house three miles away — or if someone goes to a doctor or a psychiatrist," says Nate Wessler, a staff attorney with the American Civil Liberties Union.
In Carpenter's case, the records were logs of calls he made and received. But Wessler says wireless data now includes each time a phone checks for e-mail or texts or logs on to the internet.
"A typical smartphone connects to cell towers hundreds of times a day," Carpenter's lawyers say in their written legal briefs. "The volume and precision of that data will grow steadily in coming years."
As for whether customers expect that data to be private, "it cannot be said that cellphone owners knowingly and intentionally disclose their minute-by-minute movements," the lawyers say. Using a cellphone "ought not to license total surveillance of a person's life."
The Justice Department urges the court to uphold Carpenter's conviction and rule that no search warrant was required. "Cellphone users voluntarily reveal to their providers information about their proximity to cell towers so the providers can connect to their calls," the Trump administration's legal brief says.
"Users cannot reasonably expect that the providers will not reveal that business information to the government."
In recent years, the justices have shown a willingness to extend digital-age privacy protections. The Supreme Court has ruled that police need warrants to search the contents of smartphones or to attach a GPS tracking device to a car.
The court will decide the case by late June.
Pete Williams is an NBC News correspondent who covers the Justice Department and the Supreme Court, based in Washington.