A senior Al Qaeda member, held for years at an undisclosed overseas CIA prison, was a key source of information that led investigators to alleged terror operative Jose Padilla, federal prosecutors disclosed.

The informant was Abu Zubaydah, who was transferred in September from the secret foreign prison to the U.S. detention camp at Guantanamo Bay, Cuba. Court papers filed Thursday say Zubaydah's role in the Padilla investigation was recently declassified along with that of a second Guantanamo Bay detainee identified as Binyam Muhammad.

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The pair provided information for a material witness warrant used to arrest Padilla in 2002 when he arrived at Chicago's O'Hare International Airport.

Padilla, a 36-year-old U.S. citizen, is scheduled for trial Jan. 22 with two others on charges they were part of a North American terrorism support cell that provided money, supplies and recruits to Muslim extremist causes around the world. They have pleaded not guilty.

Padilla was held without charges for 3 1/2 years as an enemy combatant until he was added late last year to the existing Miami terrorism support case.

The identities of Zubaydah and Muhammad were revealed for the first time by the government in the filing urging a federal judge not to throw out Padilla's statements and other evidence seized during an FBI interview just prior to his arrest.

Padilla's lawyers argue the evidence should be suppressed because, among other things, Zubaydah and Muhammad may have been tortured and Zubaydah was being treated with medication for gunshot wounds that raises questions about his reliability.

The government flatly rejects the allegations of torture.

Zubaydah identified a passport photograph of Padilla in early 2002 and told interrogators that Padilla and Muhammad had been working on a plot to detonate a radioactive "dirty bomb" somewhere in the United States, prosecutor Stephanie Pell wrote. U.S. officials accused Padilla of such a plot after his arrest, but it is not part of the Miami indictment.

Muhammad told investigators that he and Padilla researched the bomb plot and were trained in explosives wiring, but that Al Qaeda leaders ultimately "directed Padilla to return to the United States to conduct reconnaissance on behalf of Al Qaeda within the United States," according to the court documents.