Local police in the US may be intercepting phone calls or text messages to find suspects using a new technology tool known as Stingray.
But they are refusing to turn over details about its use or heavily censoring files when they do.
Stingray pretends it's a cellphone tower, secretly giving details of a phone's location and information about calls and text messages.
Police say it is useful for catching criminals. But civil liberties experts worry it violates Americans' privacy.
Police will not disclose details about it, including contracts with Stingray's manufacturer. They say they are protecting police tactics and commercial secrets.
Revelations about surveillance programs run by the National Security Agency have driven debate since last summer on the balance between privacy and government intrusion.
