The 20th hijacker is a possible additional terrorist in the September 11 attacks of 2001, who, for whatever reason, was not able to participate. The 20th hijacker may not have been present at the actual attack but planning and leading up to the event; the terrorist was very much involved. Working behind the scenes to provide a successful attack on the Americans. 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 only four people. The latter plane, United Airlines Flight 93, was also less successful in its mission – instead of hitting any of the obvious targets in Washington, D.C.