Twenty-five years after the terrorist attacks of Sept. 11, 2001, the greatest homeland security threats facing the United States are capable of striking American soil without a pilot.

The anniversary of the tragedy has stirred a sense of urgency in the mind of Lt. Gen. Luke Ahmann, a three-star general officer who is preparing for retirement from the U.S. Air Force. Having been in the air on the morning of the attacks and later in his career charged with commanding First Air Force’s air sovereignty operations in the continental U.S., Ahmann is keenly aware of the threats the contiguous U.S. faces from unmanned aircraft systems.