Secretary of Defense Chuck Hagel, right foreground, greets former Chairman of the Joint Chiefs of Staff U.S. Navy Adm. Michael Mullen Sept. 11, 2013, after a remembrance ceremony at the Pentagon to honor the victims of the attacks on the Pentagon Sept. 11, 2001. Hagel was joined by President Barack Obama and Chairman of the Joint Chiefs of Staff Army Gen. Martin Dempsey for a private ceremony with the victims' families. 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.