Secretary of Defense Chuck Hagel, background, participates in a 9/11 commemoration ceremony Sept. 11, 2014, at the Pentagon in Arlington, Va. Terrorists hijacked four passenger aircraft Sept. 11, 2001. Two of the aircraft were deliberately crashed into the World Trade Center in New York; one was crashed into the Pentagon; the fourth crashed near Shanksville, Pa. Nearly 3,000 people died in the attacks.