U.S. Air Force Tech. Sgts. James Gardner and Justo Scott, 633rd Security Forces Squadron flight chiefs, conduct a building sweep of the Youth Center during an active shooter exercise at Langley Air Force Base, Va., Dec. 8, 2011. The exercise helped to prepare 633rd Air Base Wing personnel for future exercises and real-world situations. (U.S. Air Force photo by Staff Sgt. Antoinette Gibson/Released)