An audio recording leaked by the hacktivist group Anonymous appears to prove that Chicago police used an IMSI catcher, commonly referred to as a Stingray, to intercept phone calls during an Eric Garner protest Thursday evening.

IMSI catchers, small devices that mimic cell phone towers, give police and government agencies the ability to capture people’s cellular data in real-time. The audio, posted to the official Anonymous Twitter account Friday, reveals a conversation between the CPIC, Chicago’s Homeland Security-run Fusion Center, and a police officer spying on an alleged protest leader.

“Yeah, just uhh, the one of the girls is kind of an organizer here, um, she’s been on her phone a lot,” an unknown government employee told CPIC. “Are you guys picking up any, uh, any, uh, information, uh, where they’re going, possibly?”

“Yea, we’re keeping an eye on it.” a CPIC employee answers. “We’ll let you know if we hear anything.”

The leaked audio comes only one day after protesters began tweeting out pictures of a mysterious government vehicle equipped with an unknown device on its hood.