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.