A 20th hijacker is a hypothetical additional terrorist in the September 11, 2001 attack who was not able to participate.

The term is somewhat misleading, as there is no evidence that al-Qaeda ever planned to have exactly twenty hijackers. There were many variations of the 9/11 plot, with the number of terrorists fluctuating with available resources and changing circumstance. In the end, there were nineteen hijackers: three of the planes were taken over by five members each, and the fourth was hijacked by only four people. Because people tend to prefer round numbers, and because one plane had fewer hijackers than the rest, the idea of a 20th hijacker came to be widely discussed.

20th hijackers

The 9/11 Commission concluded that eight members of al-Qaida, in addition to the nineteen hijackers, attempted to enter the United States to participate in the attacks.

Ramzi Binalshibh was repeatedly denied entry into the U.S and was unable to take part. Mohamed al-Kahtani was another would-be hijacker, but he was denied entry into the U.S. at Orlando International Airport in August, 2001. He was later captured and imprisoned at Guantanamo Bay.

Zacarias Moussaoui was considered as a replacement for Ziad Jarrah, who at one point threatened to withdraw from the scheme because of tensions amongst the plotters. Plans to include Moussaoui were never finalized, as the al-Qaida hierarchy had doubts about his reliability. Ultimately, Moussaoui did not play a role in the hijacking scheme. He was arrested about four weeks before the attacks.

The other al-Qaida members who attempted to take part in the attacks, but were not able, were Saeed al-Ghamdi (not to be confused with the successful hijacker of the same name), Mushabib al-Hamlan, Zakariyah Essabar, Ali Abdul Aziz Ali, and Tawfiq bin Attash. Khalid Sheikh Mohammed, the attack's mastermind, had wanted to remove at least one member — Khalid al-Mihdhar — from the operation, but he was overruled by Osama bin Laden.