Drones are well-known for their ability to hunt down suspected terrorists abroad, but they have also increasingly become a popular tool of police departments around the country.

Drones are cheaper to build and fly than helicopters, making them a cost-effective option for police departments looking to gain a bird’s eye view of a scene. But privacy groups are sounding alarm that there aren’t enough legal safeguards in place to prevent drones from being used for mass surveillance.

The privacy groups are pushing Congress to pass a law that would set nationwide restrictions on how police can use drones.

At least 13 state and local police agencies around the country have used drones in the field or in training, according to the Association for Unmanned Vehicle Systems International, an industry trade group. The Federal Aviation Administration has predicted that by the end of the decade, 30,000 commercial and government drones could be flying over U.S. skies.