There were four commercial flights from two major airlines involved in the September 11, 2001, terrorist attacks. American Airlines Flight 11 was the first to strike the North Tower of the World Trade Center, followed shortly by United Airlines Flight 175, which struck the South Tower. American Airlines Flight 77 was crashed into the Pentagon in Arlington, Virginia. The fourth aircraft, United Airlines Flight 93, crashed into a field near Shanksville, Pennsylvania, after passengers and crew fought back against the hijackers, preventing the plane from reaching its presumed target in Washington, D.C. In total, all passengers and crew on all four flights perished. This event led to the immediate grounding of all civilian aircraft in U.S. airspace and the permanent transformation of global aviation security, including the creation of the TSA and the introduction of reinforced, bulletproof cockpit doors that are now the standard in 2026.