U.S. Air Force Airman 1st Class Putthaporn Boonpreda, 633rd Civil Engineer Squadron air terminal representative, hides behind a vehicle during an active-shooter exercise at Langley Air Force Base, Va., Nov. 19, 2013. The exercise aimed to prepare Langley personnel for an active-shooter threat. (U.S. Air Force photo by Airman 1st Class Victoria H. Taylor/Released)