Updated: Local police departments across the country are tracking cellphones without a warrant, according to documents obtained by the American Civil Liberties Union.

The ACLU surveyed about 200 state and local law enforcement agencies across the country and found that only a handful of them said they obtained “warrants based on probable cause” before tracking cellphones. Most other agencies had laxer standards for tracking cellphones. In Lincoln, Neb., for instance, local police can obtain the GPS location of cellphone users “without demonstrating probable cause,” the group said in a statement.

As a result, the ACLU concludes that there are “unclear or inconsistent legal standards from town to town that frequently fall short of probable cause,” questioning whether the practice is constitutional.

In correspondence with the ACLU, the D.C. police acknowledged using cellphone data, including location information, but declined to release the legal standards it uses for obtaining cellphone location records, saying such matters were “law enforcement sensitive” and thus should not be made publicly available.

Fritz Mulhauser, staff attorney for the ACLU’s local branch for the District of Columbia, says the local police offered a few more details about their cellphone tracking policy after being pressed by a local council member. “Chief Lanier on February 24 acknowledged in a letter before the agency annual performance oversight hearing that MPD had sought 684 cell location records in the 20 months before that date,” said Mulhauser, noting that the ACLU's original query yielded “little information.”

Congress is considering a bipartisan bill that would require police to obtain a warrant to track cellphones or GPS devices, as well as banning phone companies from sharing such data without their customers’ consent. “The lack of legal clarity surrounding the use of electronically obtained location data, also known as geolocation information, means that there are no clear rules for how this data can be used, accessed or sold by law enforcement, commercial entities or private citizens,” Sen. Ron Wyden (D-Ore.), a co-sponsor of the bill, said in a news release last June when he introduced the bill.

This post has been updated since it was first published.