The document summarizes the events of September 11, 2001 terrorist attacks in the United States. Four commercial airplanes were hijacked by Al-Qaeda terrorists and deliberately crashed into buildings, killing nearly 3,000 people. Two planes crashed into the twin towers of the World Trade Center, a third into the Pentagon, and the fourth crashed in a field in Pennsylvania. The attacks were planned and carried out by 19 Al-Qaeda terrorists mainly from Saudi Arabia, led by Osama bin Laden. The attacks triggered major U.S. initiatives to combat terrorism.