U.S. President Barack H. Obama, center, places a wreath at the Pentagon Memorial on the 13th anniversary of the attack on the Pentagon in Arlington, Va., Sept. 11, 2014. 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.