One of those who provided information against Khan, Iyman Farris, the Ohio truck driver convicted of terrorism for a plot against the Brooklyn Bridge, later claimed his statements about Khan were lies and the product of coercion and deceit.
Majid Kahn's father also disavowed statements to the FBI. He submitted a letter to the War Crimes Tribunal that said:
"Anything we may have said about Majid Khan was simply out of shock because we only knew that Majid had disappeared and was pure speculation based on what FBI agents in the United States told us and pressured us to say."
As to another detainee who reportedly gave information against Kahn, Saifullah Paracha, see this article by Andy Worthington.
Writing from Guantánamo, Saifullah Paracha also refuted the claims against Majid Khan (and, by extension, against both himself and his son Uzair), confirming that he did not know that either Khan (or Ammar al-Baluchi, who introduced Khan to him in Pakistan) were members of al-Qaeda.
The Washington Post wrote this long profile on Majid Kahn in 2006 after his transfer to Gitmo.
Imprisoned since age 23, and now 31, the Government is seeking a life sentence for Khan. He's charged with conspiracy, murder and attempted murder in violation of the law of war, providing material support for terrorism, and spying. The charges are here. (Scroll to his name and then click on the docket. There is no direct link.) The transcript of his 2007 Combatant Status Review Tribunal hearing is here.
Another facet of the case: Back in 2006, the Bush Administration asked the federal court to ban Majid Kahn from meeting with this lawyers on the grounds that he might reveal to them details of his incarceration and interrogation in overseas prisons. It claimed that information was classified. The Government's pleading is here.
9. ....Prior to his current DoD custody, Majid Khan was held in the custody of the CIA in that agency’s high-value terrorist detainee program. See Dorn Decl. ¶¶ 7...
10. Further, because of Khan’s involvement in the high-value terrorist detainee program, it is likely he will possess, and may be able to transmit to counsel, information that would be classified at the TOP SECRET//SCI level, such as detention locations and other operational details, or information that would warrant equivalent treatment or other special handling while Khan remains in United States’ custody.
The documents in Khan's habeas lawsuit and lawsuit over his alleged torture are here.