A Chicago company has marketed a tool using text, photos and videos gleaned from major social media companies to aid law enforcement surveillance of protesters, civil liberties activists say.

The company, called Geofeedia, used data from Facebook, Twitter and Instagram, as well as nine other social media networks, to let users search for social media content in a specific location, as opposed to searching by words or hashtags that would be less likely to reveal an exact location.

Geofeedia marketed its abilities to law enforcement agencies and has signed up more than 500 such clients, according to an email obtained by the American Civil Liberties Union. In one document posted by the organization, as part of a report released on Tuesday, the company appears to point to how officials in Baltimore, with Geofeedia’s help, were able to monitor and respond to the violent protests that broke out after Freddie Gray died in police custody in April 2015.

Geofeedia appears to have used programs that Facebook, Twitter and other social media companies offered that allow app makers or advertising companies to create third-party tools, like ways for publishers to see where their stories are being shared on social media.