The 20th hijacker is a possible additional terrorist in the September 11 attacks of 2001 who, for unknown reasons, was unable to participate. The 20th hijacker, though not present during the actual attacks, is said to have been deeply involved in the preparations. There were many variations of the 9/11 plot, with the number of terrorists fluctuating with available resources and changing circumstances. In the end, there were 19 hijackers: three of the planes were taken over by five members each and the fourth was hijacked by four people. The latter plane, United Airlines Flight 93, crashed into a field near Shanksville, Pennsylvania, due to the resistance from passengers before it could reach its target in Washington, D.C.