Four planes were hijacked by terrorists on September 11, 2001. Two of these planes, American Airlines Flight 11 and United Airlines Flight 175, were flown into the North and South Towers of the World Trade Center, causing both towers to collapse. Another plane, American Airlines Flight 77, crashed into the Pentagon. The fourth plane, United Airlines Flight 93, crashed into a field in Pennsylvania after passengers attempted to regain control from the hijackers.