A powerful new surveillance tool being adopted by police departments across the country comes with an unusual requirement: To buy it, law enforcement officials must sign a nondisclosure agreement preventing them from saying almost anything about the technology. Any disclosure about the technology, which tracks cellphones and is often called StingRay, could allow criminals and terrorists to circumvent it, the F.B.I. has said in an affidavit. But the tool is adopted in such secrecy that communities are not always sure what they are buying or whether the technology could raise serious privacy concerns. What has opponents particularly concerned about StingRay is that the technology, unlike other phone surveillance methods, can also scan all the cellphones in the area where it is being used, not just the target phone. It's scanning the area. What is the government doing with that information?" said Linda Lye, a lawyer for the American Civil Liberties Union of Northern California, which in 2013 sued the Justice Department to force it to disclose more about the technology. In November, in a response to the lawsuit, the government said it had asked the courts to allow the technology to capture content, not just identify subscriber location.