The Obama administration has been quietly advising local police not to disclose details about surveillance technology they are using to sweep up basic cellphone data from entire neighborhoods, The Associated Press has learned.

Citing security reasons, the U.S. has intervened in routine state public records cases and criminal trials regarding use of the technology. This has resulted in police departments withholding materials or heavily censoring documents in rare instances when they disclose any about the purchase and use of such powerful surveillance equipment.

Federal involvement in local open records proceedings is unusual. It comes at a time when President Barack Obama has said he welcomes a debate on government surveillance and called for more transparency about spying in the wake of disclosures about classified federal surveillance programs.

One well-known type of this surveillance equipment is known as a Stingray, an innovative way for law enforcement to track cellphones used by suspects and gather evidence. The equipment tricks cellphones into identifying their owners’ account information and transmitting data to police as if it were a phone company’s tower. That allows police to obtain cellphone information without having to ask for help from service providers, such as Verizon or AT&T, and can locate a phone without the user even making a call or sending a text message.

But without more details about how the technology works and under what circumstances it’s used, it’s unclear whether the technology might violate a person’s constitutional rights or whether it’s a good investment of taxpayer dollars.