The attacks began at 8:46 a.m. A second plane, United Airlines Flight 175, struck the South Tower at 9:03 a.m. Both towers caught fire and collapsed within hours. At 9:37 a.m., American Airlines Flight 77 crashed into the Pentagon, the U.S. Department of Defense's headquarters in Arlington, Virginia. The fourth plane, United Airlines Flight 93, was intended for another target, possibly the U.S. Capitol or the White House, but crashed into a field in Pennsylvania after passengers attempted to overpower the hijackers.