Investigators seeking the location history of an armed robbery suspect's cellphone aren't required to obtain a search warrant before compelling the carrier to turn over the information, a federal judge has ruled.

The decision, issued by US District Judge Royce C. Lamberth of the District of Columbia, said the Stored Communications Act doesn't require investigators to get a warrant based on probable cause to access the suspect's location history pulled from cellphone towers.

The ruling was based on his interpretation of cellphone calls as “wire communications”. Under the statute, records involving those communications are subject to a less burdensome standard requiring a showing that the contents are material to an ongoing criminal investigation.

The ruling, which was unsealed on Wednesday, came as a surprise to civil liberties advocates because it disregarded a federal appeals court ruling from last year that soundly rejected US government claims that it didn't need a search warrant to surveil suspects using global positioning system location-tracking devices. The appeals court judges in that case, known as US v. Maynard, said the surveillance of the suspect's movements were so prolonged and extensive that it was barred by the US Constitution's prohibition against unreasonable searches.

In his ruling, Lamberth said so-called CSLI, or cell-site location information, was significantly more limited and therefore the Maynard decision didn't apply.

“Disclosure of historical CSLI for limited numbers of specific calls, on the other hand, does not paint such a detailed portrait of an individual's life,” he wrote. “Historical CSLI like that sought by the government here does not provide a record of a cell phone user's each and every destination, or the length of time he remains there. Indeed, historical CSLI reveals only an approximate position from which a user placed a call, and is silent as to the duration spent in transit from one place or another.”

Lamberth went on to say that under previous court rulings governing wire communications, there is no reasonable expectation of privacy in the numbers a customer dials using a landline. In the same vein, cellular customers have no privacy expectation for data their handsets transmit to nearby towers, he concluded.

Lamberth's ruling also sharply contrasts with a decision in a third case that held that CSLI provides such an intimate portrait of a customer's life that government investigators must get a warrant before obtaining it. That ruling, which borrowed heavily from Maynard, essentially found that cellphones were tantamount to tracking devices because they recorded a user's location each time he issued or received a call or text message.

Civil liberties advocates have warned that Lamberth's decision could erode people's rights to be free from unreasonable surveillance, should it be adopted widely.

“Using a cell phone, even if you know that the phone company has access to your location, shouldn’t mean that the government has a right to access this sensitive information about who you are and where you go without going to a judge and getting a warrant,” Chris Conley, of the American Civil Liberties Union of Northern California blogged. “That’s especially true when your carrier knows where you[r] phone is any time it’s turned on (even if you’re not actively using it) and retains this information for longer than you might expect.”

The dueling interpretations of the Stored Communications Act and the privacy expectations relating to historical cellphone location data are sure to be repeated. As previously reported, the US Supreme Court has agreed to decide the Maynard case. Oral arguments in the case, which is now known as US v. Jones (PDF), are scheduled for November 8.

A PDF of Lamberth's opinion is here. ®