The Sept. 11 commission report described Mohamedou Ould Salahi as a significant al Qaeda operative who instructed hijackers how to reach Afghanistan to train for jihad.
Salahi says he falsely admitted under abusive interrogation to arranging travel for some of the hijackers.
Salahi has been held without charge for eight years at the Navy-run prison at Guantanamo Bay, Cuba, and remains there as lawyers prepare to argue over his release before a three-judge panel of the U.S. Court of Appeals in Washington on Friday.
U.S. District Judge James Robertson ruled this spring that the evidence against Salahi was "tainted by coercion and mistreatment" and based on classified material that could not support a criminal prosecution.
"The government's case relies heavily on statements made by Salahi himself, but the reliability of those statements - most of them now retracted by Salahi - is open to question," the judge wrote in his order.
Salahi says he was held in isolation for months, kept in a freezing cold cell, shackled to the floor, deprived of food, made to drink salt water, forced to stand in a room with strobe lights and heavy metal music for hours at a time, threatened with harm to his family, forbidden from praying, beaten and deprived of sleep. His abuse was documented in a 2009 report by the Senate Armed Services Committee that investigated allegations of detainee abuse at Guantanamo.
Justice Department attorneys argue that Salahi was a recruiter for al Qaeda who in October 1999 encouraged Ramzi bin al Shibh, Marwan al Shehhi and Ziad Jarrah to join. Al Shehhi and Jarrah became two of the hijackers and Bin al Shibh helped coordinate the 9/11 plot.
Salahi, born in 1970, admitted that while he was an electrical engineering student at the University of Duisberg in Germany in 1990, he traveled to Afghanistan and trained to fight jihad against communists. But he argued that he stopped fighting for al Qaeda in 1992 before the organization turned against the United States.
He was arrested in his home country of Mauritania 18 days after the terrorist attacks of Sept. 11, 2001. His lawyers say he was sent to Jordan and abused for eight months before being moved to Bagram Air Field in Afghanistan and finally to Guantanamo in 2002.
Robertson said Salahi was subjected to "extensive and severe mistreatment" at Guantanamo from mid-June 2003 to September 2003.
Robertson said that under coercive interrogation, Salahi confessed to arranging travel for several of the 9/11 hijackers and justified his assistance as "just" jihad. But he later said he did nothing more than give bin al Shibh and his friends - not the hijackers - lodging for one night two years before the attacks.
Salahi admits that he stayed in touch with friends who continued to support al Qaeda, including his brother-in-law, who was a high-ranking spiritual adviser to al Qaeda leader Osama bin Laden.
Robertson said that although the evidence shows Salahi was an al Qaeda sympathizer who gave sporadic support to its members, he would not allow Salahi to be imprisoned indefinitely on suspicion that he could become a terrorist upon his release.
By Associated Press Writer Nedra Pickler