Drones are a top threat to the public at FIFA World Cup games in dozens of U.S. cities this summer, and authorities acknowledged that they cannot lock down all of them, according to FIFA and U.S. officials.

Law enforcement and security officials tasked with maintaining public safety at the games starting in June told lawmakers in Washington on Tuesday that drones used to surveil the public, as well as carry out attacks from the sky, are a serious cause for concern as they plan countermeasures, according to Mike Sena, president of the National Fusion Center Association.