United Flight 93
Credit: ASSOCIATED PRESS
Pennsylvania state troopers salute as buses carring family members arrive at the crash site of United Flight 93, in Shanksville, Pa., Monday Sept. 17. 2001. The plane, which government officials suspect the hijackers intended for a high-profile target in Washington, was the fourth to crash in a coordinated terrorist attack that killed thousands, and the only one that didn't take lives on the ground.