The largest police force in the United Kingdom is using surveillance technology that enables them to monitor cell phone conversations and sweep up large amounts of data without their knowledge, according to a Sky News report. The controversial technology, known as stingrays or ISMI catchers, has been widely adopted throughout the U.S.

Sky News deployed software made by GMSK Cryptophone, a German phone encryption company, to detect more than 20 instances of stingray use over three weeks in London. By mimicking a cell tower signal, stingrays dupe potentially tens of thousands in a given area into connecting to the police device instead of an actual tower. That connection makes it possible for the stingray owner to monitor snippets of a person's conversation, track the phone's physical location, sweep up vast amounts of metadata on text messages and call logs and collect other information that typically requires a warrant.

London's Metropolitan police obtained a stingray from the Leeds-based company Datong plc, which also sells the surveillance systems to the U.S. Secret Service, the U.K. Ministry of Defense and governments in the Middle East, according to a 2011 report in the Guardian. But it might be impossible to know how many times the stingrays have been deployed because, according to a report from the group Privacy International, U.K. police could argue that use is protected by the 1997 Police Act, which allows for "property interference" by investigators.

“With IMSI catchers, it's very difficult for them to be used in a targeted manner,” Eric King of Privacy International told Sky News. “In an urban space, thousands of people's mobile phones would be swept up in that dragnet. What they do with that data, we don't know.”

Bernard Hogan-Howe, commissioner of the Metropolitan police, refused to delve into any specifics with Sky News, saying “We're not going to talk about it, because the only people who benefit are the other side, and I see no reason in giving away that sort of thing.”