A federal magistrate has ruled that information pulled from cellphone towers provides such an intimate portrait of a customer's life that government investigators must get a warrant before obtaining it.

The ruling by Magistrate Judge James Orenstein of the US District Court for Eastern New York is a major victory for civil liberties advocates, who have long warned that so-called historical cell-site information gives the government the ability to track users' location each time they make a call or send a text message. It follows an appeals court decision earlier this month that a drug trafficking suspect's constitutional rights were violated when the FBI surreptitiously attached a global positioning system to his car without a warrant.

The ruling in that case, known as US v. Maynard, weighed heavily in Orenstein's reasoning.

“The decision in Maynard is just one of several rulings in recent years reflecting a growing recognition, at least in some courts, that technology has progressed to a point where a person who wishes to partake in the social, cultural, and political affairs of our society has no realistic choice but to expose to others, if not to the public as a whole, a broad range of conduct and communications that would previously have been deemed unquestionably private,” he wrote.

The decision, which was released on Friday, denied government investigators' request for an order requiring Sprint Nextel to turn over cell-site information for a handset used by an individual under scrutiny in a criminal probe. Prosecutors, who based their request on the Stored Communications Act, argued that they didn't need a warrant based on a 1983 ruling in a case known as US v. Knotts, which held that location tracking outside the home is analogous to physical surveillance.

In light of more recent decisions, Orenstein disagreed.

“As a result of such decisions, I believe that magistrate judges presented with ex parte requests for authority to deploy various forms of warrantless location-tracking must carefully re-examine the constitutionality of such investigative techniques, and that it is no longer enough to dismiss the need for such analysis by relying on cases such as Knotts.” he said. “For the reasons discussed below, I now conclude that the Fourth Amendment prohibits as an unreasonable search and seizure the order the government now seeks in the absence of a showing of 'probable cause, supported by Oath or affirmation.'”

The 30-page ruling rejected government arguments that there can be no reasonable expectation that cell-site information is private. It cites assurances that carriers such as Verizon Wireless offer that customers are in control of location-tracking features and that their concerns about how their information is shared is taken seriously.

“Simply put, there is no reason to think that the advance of technology brings with it an expectation that privacy is lost,” he wrote. “Rather, I assume that it serves only to increase awareness of the importance of privacy and to whet the appetite for ways to manage it.”

A PDF of the decision is here and you can find the ACLU's take here. ®