In a Freedom of Information Act request, the ACLU has asked the Justice Department to turn over documents that might shed light on how federal officials have reportedly used locator signals from cell phones to track people's movements.
The practice was first reported by the Washington Post last week, which called it "routine," and noted that prosecutors sometimes obtained the tracking information without demonstrating probable cause.
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"Giving the government the power to track and store a person's whereabouts at any given moment without probable cause is a serious intrusion on our personal privacy," said Catherine Crump, staff attorney for the ACLU, explaining the reason for her group's request. "Carrying around a cell phone should not make a person susceptible to that kind of surveillance."
A Justice Department attorney told the Post that federal prosecutors were expected to follow the department's policy, which was to obtain a warrant based on probable cause in order to obtain the tracking information.
"Law enforcement has absolutely no interest in tracking the locations of law-abiding citizens," Justice Department spokesman Dean Boyd told ABC News. Last year, the practice helped win the confession of a man who kidnapped and raped a young girl, and locate the girl where the culprit had left her, he said.
This post has been updated.