On September 11, 2001, a series of coordinated terrorist attacks were carried out by the Islamist extremist group al-Qaeda against the United States. Four commercial airplanes were hijacked; two were flown into the Twin Towers of the World Trade Center in New York City, causing both buildings to collapse. Another plane struck the Pentagon, while the fourth, United Airlines Flight 93, crashed in Pennsylvania after passengers attempted to regain control. This tragic event resulted in thousands of deaths, extensive destruction, and significant changes in U.S. domestic and foreign policies, leading to the War on Terror and increased security measures nationwide.