The September 11 attacks occurred in the year 2001. On September 11, 2001, nineteen terrorists associated with al-Qaeda hijacked four commercial airplanes and carried out coordinated suicide attacks against the United States. Two planes were flown into the Twin Towers of the World Trade Center in New York City, one into the Pentagon in Washington, D.C., and the fourth crashed in Pennsylvania after passengers intervened. This event is the deadliest terrorist attack in U.S. history, killing 2,977 people