Federal investigators used a "stingray" device to track Michael Cohen, as they tried to pin down exactly where he was staying at the Loew's Regency Hotel in New York City, according to newly unsealed documents.

The stingray is a spoofing device used for surveillance. Nearby cell phones connect to the stingray device, thinking it is a cell phone tower. Then the people operating the device can snoop on the traffic and collect information.

Federal agents used the device, which they called a "triggerfish" in the documents, to track Cohen’s location in April 2018. They figured out which hotel room he was staying at based on the pings from his phone, according to the court filings. Prosecutors said in the court filings that they “sought and obtained” permission to use the stingray technique.

One day later, the FBI raided Cohen’s hotel room, and other properties.