The man alleged to be the real 20th hijacker of the September 11 terror attacks is imprisoned in Guantanamo Bay, it has emerged.

Muhammad al-Qahtani, a Saudi, was arrested after US immigration authorities refused to allow him to enter the country at Orlando airport in Florida, before the suicide hijackings.

Testimony in the trial of Zacarias Moussaoui quoted Khalid Shaikh Muhammad, the mastermind of the 9/11 terror attacks who has been in US custody for several years, describing al-Qahtani as the last hijacker for the mission who would "complete the group".

Nineteen hijackers boarded four planes on the morning of September 11, 2001. Three planes each with five terrorists on board struck their intended targets, but the fourth, United Airlines flight 93, only had four hijackers. They were forced to dive the aircraft into the ground short of their target - probably the White House - after a counter-attack by the passengers.

Claims by Moussaoui himself at his trial that he was intended to be one of the 9/11 hijackers have been discounted by intelligence sources, although he may still be condemned to death for failing to tell the security services what he knew about the plot.

Although al-Qahtani's presence at the prison camp in Cuba had been reported, the military had previously declined to confirm it.

News of his whereabouts emerged on a Pentagon list of the hundreds of detainees who have been held at the Guantanamo Bay prison, published in response to a Freedom of Information Act lawsuit by the US-based Associated Press news agency. Al-Qahtani appears as number 50 on a list of 558 names.

The list is the first official roster of Guantanamo detainees who passed through the Combatant Status Review Tribunal process in 2004 and 2005 to determine whether they should be deemed "enemy combatants." Nearly all the men were ruled to be enemy fighters, but only a handful have so far faced formal charges, despite four years or more in custody.

In all, 558 people were named in the list provided by the Pentagon, some of whom have already been released. Some names are familiar, such as David Hicks, a Muslim from Australia charged with fighting US and coalition forces in Afghanistan, who is seeking to get a British passport after his own government refused to request his release.

Hicks is one of 10 detainees selected to be tried by a military tribunal on charges of attempted murder, aiding the enemy and conspiracy to commit terrorism. He allegedly fought for the Taleban, and according to reports has admitted training with British Islamic extremists, including the would-be shoe-bomber Richard Reid, who was convicted of trying to blow up a trans-Atlantic airliner in 2001.

The list also includes top former Taleban officials, such as Mullah Mohammed Fazil, the ousted regime’s former Defence Ministry chief of staff; Abdul Haq Wasiq and Gholam Ruhani, Taleban intelligence officials who are believed to still be in custody; and Abdul Salam Zaeef, the Taleban’s former ambassador to Pakistan, who was released in late 2005.

Others on the list, such as an Afghan identified only as "Commander Chaman," remain mysterious.

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