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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. One plane, United Airlines Flight 93, had fewer hijackers than the rest and was also less successful in its mission—instead of hitting any of the obvious targets in Washington, D.C., it hit a hillside in Pennsylvania, due to resistance from passengers. Thus the idea of a 20th hijacker came to be widely discussed.
The 9/11 Commission concluded that Khalid Sheikh Mohammed had intended to have as many as 25 or 26 hijackers for the plot. It was also reported that 14 members of al-Qaeda, in addition to the 19 known hijackers, had attempted to enter the United States to participate in the attacks.
Mohamed al-Kahtani, a Saudi Arabian citizen, is often referred to as the 20th hijacker. José Meléndez-Pérez, a US Immigration inspector at Orlando International Airport refused his entry into the US in August 2001. He was later captured in Afghanistan and imprisoned at the US military prison known as Camp X-Ray at Guantanamo Bay, Cuba. In January 2009, Susan J. Crawford asserted that Kahtani's interrogation at Camp X-Ray amounted to torture.
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-Qaeda 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-Qaeda members who allegedly attempted, but were not able, to take part in the attacks were Saeed al-Ghamdi (not to be confused with the successful hijacker of the same name), Tawfiq bin Attash, Ali Abdul Aziz Ali, Mushabib al-Hamlan, Zakariyah Essabar, Saeed Ahmad al-Zahrani, Ali Abd al-Rahman al-Faqasi al-Ghamdi, Saeed al-Baluchi, Qutaybah al-Najdi, Zuhair al-Thubaiti, and Saud al-Rashi. Khalid Sheikh Mohammed, the attack's alleged mastermind, had wanted to remove at least one member—Khalid al-Mihdhar—from the operation, but he was overruled by Osama bin Laden.
According to the BBC, Fawaz al-Nashimi claimed to have been the "20th hijacker". An Al-Qaeda video has been released from a US intelligence organization, showing al-Nashimi justifying attacks on the west. The U.S dismissed al-Nashimi's claims as propaganda. He was also known as Turki bin Fuheid al-Muteiry and took part in a May 29, 2004 attack on oil facilities in Khobar, Saudi Arabia. He was killed in a June 2004 shootout with Saudi Arabian security forces.
In fiction 
The Saudi novelist Abdullah Thabit has written a bestselling novel titled Terrorist Number 20 (2006). The book recalls his teenage years as a religious extremist and was inspired in part by Ahmad Alnami, one of the 9/11 hijackers and a fellow resident of Abha who was vaguely familiar to Thabit. In April 2006, three months after the release of the book, Thabit was forced to move from Abha to Jeddah with his family after receiving death threats from Islamic radicals.
See also 
- USS Cole bombing mastermind -- multiple individuals were alleged to be the mastermind of the USS Cole bombing
- Smith, Elliot Blair (September 17, 2002). "U.S. blood 'all over his hands'". USA Today. Retrieved February 13, 2007.
- Shenon, Philip (January 27, 2004). "Panel Says a Deported Saudi Was Likely '20th' Hijacker". The New York Times. Retrieved February 13, 2007.[dead link]
- Kean, Thomas; et al. (July 22, 2004). Final Report of the National Commission on Terrorist Attacks Upon the United States (PDF). US Government Printing Office. p. 237. ISBN 0-16-072304-3. "Mihdhar complained about life in the United States. He met with KSM, who remained annoyed at his decision to go AWOL. But KSM's desire to drop him from the operation yielded to Bin Ladin's insistence to keep him."
- "'Al-Qaeda video' of 20th hijacker". BBC. June 21, 2006.
- Shrader, Katherine (June 21, 2006). "al-Qaida Video Shows Alleged 20th Hijacker". Associated Press.
- Profile of Thabit in the Washington Post, July 2006