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.

The confidentiality has elevated the stakes in a longstanding debate about the public disclosure of government practices versus law enforcement’s desire to keep its methods confidential. While companies routinely require nondisclosure agreements for technical products, legal experts say these agreements raise questions and are unusual given the privacy and even constitutional issues at stake.

“It might be a totally legitimate business interest, or maybe they’re trying to keep people from realizing there are bigger privacy problems,” said Orin S. Kerr, a privacy law expert at George Washington University. “What’s the secret that they’re trying to hide?”