A new map released Thursday by the American Civil Liberties Union shows that fake cell towers, also known as stingrays, are used by state and local law enforcement in 15 states.

Police departments in Baltimore, Chicago, Houston, Tucson, Los Angeles, and even Anchorage, among others, have been confirmed to use the devices. Beyond those states, 12 federal law enforcement agencies, ranging from the FBI to the National Security Agency, also employ them.

Relatively little is known about precisely how police decide when and where to deploy them, but stingrays are used to track targeted phones and can also be used to intercept calls and text messages. However, privacy advocates worry that while the devices go after specific targets, they also often capture data of nearby unrelated people.

While "Stringray" is a trademarked name of the Harris Corporation and particular product, it has entered the technical lexicon as a generic term like Kleenex or Xerox. In nearly every sales agreement, Harris has required law enforcement agencies to sign nondisclosure agreements forbidding them from discussing whether or not an agency even possesses such a device, much less describing its capabilities.

The ACLU’s Chris Soghoian recently noted in an academic paper that other companies from other countries, including China, Turkey, the Netherlands, and Canada, manufacture similar devices.

The new map comes just two days after an activist in Chicago sued the Chicago Police Department for failing to release stingray-related information requested under the Illinois Freedom of Information Act. ACLU attorney Nathan Wessler also penned an op-ed for Al-Jazeera America on Thursday, in which he outlined his concerns about the use of stingrays by local law enforcement.

He wrote: