BARKSDALE AIR FORCE BASE, La. --   Senior Airman Kyle Harper, 2nd Security Forces Squadron entry controller, prepares for an active shooter exercise at Barksdale Air Force Base, La., July 18, 2016. During the training, defenders clear a building room by room, looking for the shooter. Once the shooter has been located and neutralized, the team does a second building sweep to make sure no threats remain. (U.S. Air Force photo/Senior Airman Curt Beach)