From left, President Barack Obama, Secretary of Defense Chuck Hagel and Chairman of the Joint Chiefs of Staff U.S. Army Gen. Martin Dempsey stand for the national anthem Sept. 11, 2013, during a remembrance ceremony at the Pentagon for victims of the attacks on the Pentagon Sept. 11, 2001. The three leaders joined family members of those killed 12 years ago for a private ceremony. 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.