Several law enforcement agencies around the Bay Area are using a controversial surveillance technology to track people and collect data in real time, according to KQED. “StingRays” act like cell towers, attracting all the surrounding wireless devices on the same network and retrieving their data. Police departments in San Francisco, Oakland, San Jose and Fremont are all using these devices.

According to Hanni Fakhoury of the Electronic Frontier Foundation, which advocates for civil rights in the digital world, a StingRay can capture conversation (although law enforcement officials offer assurances that they do not use this capability) and the metadata or information about how you communicate.

Read more of Fakhoury’s take on StingRay technology, including the need for warrants, here.