During the 9/11 attacks, four commercial planes were hijacked by terrorists. These planes were American Airlines Flight 11, United Airlines Flight 175, American Airlines Flight 77, and United Airlines Flight 93. Three of these planes were flown into the World Trade Center's Twin Towers and the Pentagon, while the fourth crashed in a field in Pennsylvania after passengers attempted to retake control of the plane. All four hijackings were carried out by a total of 19 terrorists.