A document obtained by BuzzFeed News shows the US Department of Homeland Security used secretive cell phone–tracking devices nationwide more than 1,800 times from 2013 to 2017.

The information, obtained through a Freedom of Information Act request, shows that Homeland Security Investigations, a major investigative arm of DHS, used what's known as cell-site simulator over-the-air technology 1,885 times from Jan. 1, 2013, to Oct. 11, 2017 throughout the US.

HSI investigates cross-border crimes that include immigration violations, financial fraud, and drug smuggling. DHS bills HSI as being its largest investigative presence outside the United States and having one of the largest international footprints of any US law enforcement agency.

How law enforcement agencies make use of the devices, including how often and under what circumstances, has largely been shrouded in secrecy.

Cell-site simulators pretend to be cell phone towers and trick nearby phones into connecting to them instead of to a legitimate signal. Authorities can use the devices, often referred to as a “Stingray" because of an early model of the cell-site simulator, to track down a suspect if they already have their phone information.

Groups such as the ACLU and Electronic Freedom Foundation say the devices disrupt 911 calls and invade the privacy of everyone who happens to be near the cell-site simulator by collecting metadata from their phones, too.

Dave Maass, a researcher with EFF, said the new 1,885 figure gives some sense of scale for how many times DHS uses the devices. It also leads to more questions.

“It’s hard for us to know if that number is surprising or not, they’ve been so opaque about how cell-site simulators are used,” Maass told BuzzFeed News. “I’m curious to know how many times a month they use it, what regions they’re mostly deployed in, and if they let local police borrow them.”

DHS did not respond to a request for comment.

In 2016, the New York Times reported that the NYPD used the covert devices on at least 1,016 occasions from 2008 to 2015. The US Marshals Service used the surveillance tools nearly 6,000 times throughout the United States, USA Today reported last year, adding that until then Baltimore police had been the most prolific user. That was based on testimony from a detective testified that police had used the device 4,300 times.