Secretary of Defense Chuck Hagel, foreground left, exchanges greetings with President Barack Obama Sept. 11, 2014, before a ceremony at the Pentagon in Arlington, Va., to mark the anniversary of 9/11. 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.