New York, Los Angeles, Chicago and Las Vegas are among police departments across the US using a highly secretive technology developed for the military that can track suspects by using the signals emitted by their cellphones.

Civil liberties and privacy groups are increasingly raising objections to the suitcase-sized devices known as StingRays or cell site simulators that can sweep up cellphone data from an entire neighbourhood by mimicking cell towers. Police can find the location of a phone without the user calling or texting. Some versions of the technology can intercept texts and calls, or pull information stored on the phones.

Part of the problem, privacy experts say, is the devices can also collect data from anyone within a small radius of the person being tracked.

At least 72 state and local law enforcement departments in 24 states plus 13 federal agencies use the devices. The departments must sign nondisclosure agreements overseen by the FBI. "We can't even tell how frequently they're being used," said lawyer Jerome Greco of the Legal Aid Society, which recently succeeded in blocking evidence collected with the device in a New York murder case.

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In New York, use of the technology was virtually unknown to the public until last year when the New York Civil Liberties Union forced the disclosure of records showing the NYPD had used the devices more than 1000 times since 2008. The technology helped catch suspects in kidnappings, rapes, robberies, assaults and murders.

Case law is slowly building. Two months ago, a Washington, DC, appeals court overturned a conviction on a sex assault after judges ruled a violation of the Fourth Amendment because of evidence improperly collected without a proper warrant.