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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., it crashed into a field near Shanksville, Pennsylvania, due to resistance from passengers. Thus the idea of a 20th hijacker came to be widely discussed.
Mohammed al-Qahtani, a Saudi Arabian citizen, is often referred to as the 20th hijacker. José Meléndez-Pérez, a U.S. Immigration inspector at Orlando International Airport refused his entry into the U.S. in August 2001. He was later captured in Afghanistan and imprisoned at the U.S. military prison known as Camp X-Ray at Guantanamo Bay, Cuba. In January 2009, Susan J. Crawford asserted that Qahtani's interrogation at Camp X-Ray amounted to torture.
Zacarias Moussaoui, known as the 20th hijacker, was of many Saudi Arabian men involved in the September 11th terrorism attacks. Moussaoui had been working for Osama Bin Laden for a few years prior to this attack. He entered names of donors into a database; this job was given to him because of his connection with Saudi Royalty family and his educational stance while also being able to speak English. 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.
Moussaoui is now serving a life sentence for his involvement in the 9/11 attacks. He pleaded guilty in 2005 to collaborating with the other hijackers. 
In popular culture
The Saudi Arabian novelist Abdullah Thabit wrote a 2006 novel titled Terrorist Number 20 that became a bestseller. 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.
- USS Cole bombing mastermind – Multiple individuals were alleged to be the mastermind of the USS Cole bombing.
- Sciutto, Jim (4 February 2015). "New allegations of Saudi involvement in 9/11". CNN. Retrieved 23 April 2015.
- 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 23 April 2015.
- 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.
- Hallowell, Billy (17 November 2014). "Faith Convicted Terrorist Known as the ’20th Hijacker’ Has a Stunning Claim About the 9/11 Attacks". Retrieved 23 April 2015.
- "Interview with Abdullah Thabit in Washington Post - R A Y A - agency for Arabic literature". rayaagency.org.