On the morning of September 11, 2001, 19 terrorists, working in teams of four or five, hijacked four commercial airliners. The terrorists crashed two of the planes into the World Trade Center in New York City, which eventually destroyed the structure. A third plane crashed into and seriously damaged the Pentagon in Washington, D.C., while a fourth crashed in a field in Pennsylvania. The hijackings killed nearly 3000 people.
The investigation into the attacks focused almost immediately on the activities of Osama Bin Laden, leader of the al-Qaeda terrorist organization. Investigators determined that the terrorists who staged the hijackings had lived in the United States for several months prior to the attacks. Several U.S. agencies, such as the Central Intelligence Agency (CIA), Federal Bureau of Investigation (FBI), and the Department of Defense, later fell under harsh criticism for failing to communicate effectively with one another in a manner that could have prevented the terrorism from taking place.