Rosenstein called the circumstances unique. "We have never needed such an order in any case before or since, as far as I know, so there is little reason to appeal the moot opinion," he said in a statement.

Rosenstein said "the Justice Department is not proposing to use GPS to track random citizens. The person sought in this case was a felony criminal defendant for whom authorities had an arrest warrant. … Because of the circumstances of the case … they were concerned that he would flee if they called to ask him to turn himself in."

But Gauvey wrote that the suspect had no idea a warrant had been issued. The judge repeatedly noted in her ruling that "the government has reported no attempts of the subject to flee" and she questioned authorities for labeling the man a "fugitive" — saying that implies he had been a convicted felon, rather than a person accused of an unproven offense.

The judge did write that had there been evidence of a risk of flight, the request for the warrant "would have been fairly routine."

During a hearing in U.S. District Court in Baltimore, Nathan S. Judish, a senior counsel in the computer crimes and intellectual property division within the U.S. Department of Justice, sparred with the judge over how GPS data could be obtained.

"I think we are entitled to take reasonable steps to effectuate an arrest warrant and determine the location of the person subject to the arrest warrant," Judish argued, according to a transcript of the proceeding. "It seems to me extremely reasonable. We need to go where the person is and arrest them."

Gauvey countered, saying it was "highly likely" that the government would track the suspect inside his home, where he has an expectation of privacy, as much as they would track him on a public street, where he has no expectation of privacy.

The judge said the suspect has a right to privacy not only "in his location," but also "in his movement."

Judish said a suspect wanted on a warrant enjoys less privacy than others. But Gauvey disagreed. "I don't know any authority that says that," she said.

The judge also noted that GPS devices can pinpoint a person within 10 square meters, and that the average house size is 743 square meters. She cited studies showing that 65 percent of people sleep with their cellphones next to them. She questioned prosecutors wanting "unlimited location data at any time on demand during a 30-day period."

Ward, the Johns Hopkins police expert, said that warrants seeking access to GPS data cannot define the parameters and scope of the request, and ask for permission to spy on people in places and in settings not yet known.

That, he said, makes some judges uneasy.

"Warrants involving GPS allow police to develop patterns," Ward said. "It's not only who they're calling and what they're saying, but where they're going."

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