Thousands of times a year, the nation's local police departments get phone company records allowing them to plot the movements of individual customers. The issue before the court Wednesday was whether access to that data should require a search warrant issued by a judge.
When a cellphone is used for calls or text messages, it signals the closest 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.
Let our news meet your inbox. The news and stories that matters, delivered weekday mornings.
Nate Wessler, an American Civil Liberties Union lawyer who appeared before the court, called those records "digital bread crumbs" and said they can be highly revealing.
"Access to these historical location records presents the government with a veritable time machine, the ability to press rewind on someone's life and learn where we've been far into the past," Wessler said.
The case involves 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 store at the time of the crimes.
Because the FBI did not get a search warrant, Carpenter said that evidence should be thrown out.
In past cases, the Supreme Court has ruled that people have no privacy interest in the checks they write or the phone numbers they dial from landlines, because it's understood the information will be used to conduct business — for financial transactions or to generate phone bills.
Justice Department lawyer Michael Dreeben told the court that no search warrant was needed, because cellphone customers have the same understanding, aware that the phone company will keep records of their calls, and willingly give up any claim of privacy to it.
"There is an element here of voluntariness in deciding to contract with a cell company, just like there's an element of voluntariness in getting a landline phone and making calls," Dreeben said. "And there's an element of voluntariness in signing up for a bank account and using a debit card to purchase everything in your life."
But Chief Justice John Roberts questioned how voluntarily people surrender that information. "You really don't have a choice these days if you want to have a cellphone."
After listening to the 80-minute courtroom argument, many legal experts predicted the government would lose the case.
"This is probably the most significant privacy ruling the court would make for the last 10 years, because it would finally put a barrier to the government getting intimate details about people in criminal cases," said Mark Rasch, a former Justice Department computer crimes prosecutor.
In recent years, the justices have shown a willingness to extend digital-age privacy protection, ruling that police need warrants to search through the contents of smartphones or to attach a GPS tracking device to a car.
The court will issue its decision by late June.
Pete Williams is an NBC News correspondent who covers the Justice Department and the Supreme Court, based in Washington.