From foreground, President Barack Obama and Secretary of Defense Chuck Hagel participate in a wreath-laying ceremony Sept. 11, 2014, at the Pentagon Memorial in Arlington, Va., in honor of all those who died Sept. 11, 2001. 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.