WASHINGTON — The Supreme Court agreed Monday to consider whether police need a search warrant to track the movements of cell phone users.

It's the latest plunge by the justices into the issue of digital-age privacy, at a time when 95 percent of Americans own a cell phone.

Whether the Constitution protects data revealing a cellular telephone's location is "a hugely important question," said Orin Kerr, at professor at the George Washington University's law school.

Man checking email on cell phone Sigrid Olsson / Getty Images/PhotoAlto

When a cell phone is used for calls or text messages, it signals a nearby antenna tower to connect with the telephone network. As the user travels, the call is handed off to successive cell towers, and the cell phone companies keep records of the phone numbers routed through each tower.

Related: Baltimore Case on Cellphone Tracking Echoes Broader Concerns

Police can use that information to develop a map of of a person's movements, nearly minute-by-minute. Maryland Public Defender James Wyda says the government "can turn back the clock to surveil a suspect's location and movements when the suspect was not even a suspect, and perhaps when no investigation was underway -- before a crime may have been contemplated, yet alone committed."

The court agreed to hear the case of a midwestern man, Timothy Carpenter, who was charged with committing a string of robberies in Michigan and Ohio during 2010 and 2011. The police created a plot of his movements over 127 days, which prosecutors said established that he was at the scene of several unsolved robberies.

Related: California Police Now Need a Warrant Before Searching Your Cellphone

A jury convicted him of six robberies after an FBI agent testified that the records placed him near the sites of four of them. A federal appeals court upheld the conviction, ruling that cell phone users have no reasonable expectation of privacy in the records of their cell phone usage.

The lower courts have generally said police do not need a search warrant to get cell phone locator records, relying on a Supreme Court decision from 38 years ago. The court said then that no warrant is needed to get the numbers a telephone user calls, because people have no expectation that the phone numbers they dial will remain private.

"All telephone users realize that they must 'convey' phone numbers to the telephone company, since it is through telephone company switching equipment that their calls are completed," the court said. And the phone company keeps the records anyway for billing purposes.

But the ACLU, representing Carpenter, said the reasoning of that decades-old case shouldn't apply in the digital age, because the court's earlier decision involved telephones that were hard-wired into the wall and didn't move. Getting cell phone tower location, by contrast, allows the police to reconstruct a person's movements and should require a search warrant.

Related: Secret Service Gets Warrant Exemption for Cellphone Tracking

“Because cell phone location records can reveal countless private details of our lives, police should only be able to access them by getting a warrant based on probable cause,” said the ACLU's Nathan Freed Wessler.

The Supreme Court has been gradually expanding protections for digital privacy, declaring that it's a search when police attach a GPS tracker to a person's car and ruling that police need a search warrant to search a cell phone carried by a suspect.

The court will hear this latest case in the fall.