How did the attack of september 11 happen?

On September 11, 2001, a coordinated terrorist attack was carried out by the Islamist extremist group al-Qaeda. Nineteen hijackers took control of four commercial airplanes, with two crashing into the Twin Towers of the World Trade Center in New York City, another hitting the Pentagon in Arlington, Virginia, and the fourth, United Airlines Flight 93, crashing in Pennsylvania after passengers attempted to regain control. The attacks resulted in nearly 3,000 deaths and led to significant changes in U.S. and global security policies.