Former Chairman of the Joint Chiefs of Staff Charles Q. Brown Jr. said the United States should not be “surprised” if there is a drone attack within the country due to the growing threat that they pose across the globe.The federal government has worked to improve the country’s counter-drone technology, but the proliferation and evolution of drone technology make staying ahead of the threat a game of cat and mouse. In particular, there have been recent unauthorized drone sightings over outdoor sporting events and military bases, though there has yet to be any sort of domestic drone attack.“You have this drone activity that hasn’t become kinetic, and so we can use hope as a course of action that we hope this will never happen, but when it does, you should not be surprised, and we’re not trying to put a lot of fear in here,” Brown said in reference to those sightings on Thursday at the Center for a New American Security conference.

His comments came as the World Cup has begun, which the U.S., Mexico, and Canada are jointly hosting. All 11 U.S. stadiums that will host World Cup matches have counter-drone technology, Homeland Security Secretary Markwayne Mullin told lawmakers last week.

“Drones are my biggest concern too. We have spent a tremendous amount of ability and money to be able to be very offensive with drones, but on the counter-drone measures, everybody’s a little behind,” he acknowledged in response to a question about the security surrounding the international soccer tournament.