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Federal prosecutors have now confirmed privacy advocates' suspicion that Milwaukee police used a so-called Stingray to track down a suspected probation violator by zeroing in on his cellphone.

Now the phone user, Damian Patrick, has asked for a new trial, claiming the undisclosed use of the powerful technology violated his rights against unreasonable, warrantless searches.

Patrick's case is among a few around the nation pressing for clearer rules for the growing conflict between police and privacy in the digital age.

Lawyers for Patrick, whose conviction is on appeal, have asked that a federal trial judge vacate his conviction now based on the new information, and hear new arguments why evidence against Patrick should be suppressed because of the Stingray search.

"The underlying argument is the same, that the order didn't satisfy the Fourth Amendment," said Patrick's attorney, Christopher Donovan. "This is just more, 'Hey, they lied,' another avenue" for a new trial.

Patrick, 27, was charged with being a felon in possession of a gun after police stopped a car he was in and found the weapon on the floor.

It was only during his motion to suppress the use of the gun as evidence that he and his lawyers learned police had not found him based on an anonymous tip but, they said then, by tracking his cellphone with information from his service provider.

Patrick made a conditional guilty plea, was sentenced to 57 months and then appealed the denial of his suppression motion.

As part of that appeal, the American Civil Liberties Union and the Electronic Frontier Foundation, a digital privacy advocacy group, filed friend of the court briefs that suggested police probably used a Stingray to find Patrick.

Last month, in a letter to defense attorneys, federal prosecutors confirmed that Milwaukee police had used a Stingray to find Patrick on the day he was arrested in 2013.

"It's significant that they admitted it, and only, essentially, after they were caught by outside organizations," said Nathan Wessler, a lawyer with the ACLU's speech, privacy and technology project.

"There are probably hundreds of other cases in Milwaukee where this disclosure should happen as well."

Nondisclosure promises

A Stingray mimics a cellphone tower and draws all phones within its range to connect through it. The suitcase-size devices can find phones even when they're not in use or when they have location privacy enabled. They can locate a phone within a few feet but also pull in information from all non-target phones within the Stingray's reach.

To use the technology, local law enforcement agencies have to promise the FBI they will not disclose that use, even to judges who might inquire, without a written OK from the FBI.

Stingray use rapidly expanded in the past decade, raising thorny constitutional questions about whether a search warrant should be required — at least in the few cases where Stingray use was discovered.

After the Milwaukee Journal Sentinel reported on Patrick's case, U.S. Rep. Jim Sensenbrenner (R-Wis.) and a House of Representatives colleague from Texas in February asked FBI Director James Comey to clarify whether the FBI considers state and local law enforcement agencies bound by the nondisclosure agreements regarding Stingray technology.

They also asked how many such agreements exist, how many times the FBI ordered criminal prosecutions dismissed rather than disclose the use of Stingrays and whether the FBI has similar agreements with local police for use of other technology.

A Sensensbrenner spokeswoman said earlier this month that the FBI had not responded by the requested March 26 deadline.

Concern about the secrecy surrounding police use of cell-site simulators has been growing. Last month, a Maryland appeals court came down hard on Baltimore police for trying to hide Stingray use from a court.

The ruling upheld a trial judge's decision to suppress all evidence against a shooting suspect found as a result of tracing him via live cellphone tracking with a Stingray.

The court flatly rejected Maryland's argument that anyone who turns on a cellphone is voluntarily sharing their location with police, and required police to get a search warrant to use Stingrays and similar technology, as well as fully explain to a court what a device does.

"I hope that signals the momentum is shifting," said Wessler, the ACLU lawyer. "If I were the FBI, I would be scrambling to fix this."