"Al-Marri was sent to the United States as a facilitator for other al Qaeda individuals who would come in to conduct follow-on attacks," then-Attorney General John Ashcroft told ABC News in an exclusive interview in June 2003.
The government arrested al-Marri in December 2001, then charged him in a criminal complaint filed in December 2002. The FBI alleged he repeatedly attempted to contact an al Qaeda leader suspected of helping to finance the 9/11 attacks.
The Justice Department also claimed in the statement it released Monday that "In the summer of 2001, he [al-Marri] met with Khalid Shaykh Muhammed, the mastermind of the Sept. 11 attacks and entered the United States just before Sept. 11 to serve as an al Qaeda sleeper agent and to explore methods of disrupting the U.S. financial system."
When FBI agents searched al-Marri's West Peoria, Ill., home after his arrest, they said they found dozens of credit card numbers on his laptop, information on toxic chemicals, and lectures from Osama bin Laden and other top al Qaeda associates. They also said they'd discovered an almanac with a variety of potential targets marked, including dams and railroads.
Al-Marri entered the U.S. legally on Sept. 10, 2001.
Though al-Marri is a citizen of Qatar, not the United States, the court said he could not be held indefinitely without the right to due process.
The court concluded that "the president claims power that far exceeds that granted him by the Constitution."
But one judge on the appeals panel did not agree with the majority opinion.
"I believe the district court correctly concluded that the president had the authority to detain al-Marri as an enemy combatant or belligerent," Judge Henry E. Hudson wrote in his dissent.
"Although al-Marri was not personally engaged in armed conflict with U.S. forces, he is the type of stealth warrior used by al Qaeda to perpetrate terrorist acts against the United States."
Though the two judges who made up the majority rebuked the standing policy, their opinion noted that al-Marri shouldn't necessarily go free, but instead, be tried by a civilian court.
"If the government accurately describes al-Marri's conduct, he has committed grave crimes. But we have found no authority for holding that the evidence offered by the government affords a basis for treating al-Marri as an enemy combatant, or as anything other than a civilian," Judge Diana Gribbon Motz wrote.
"This does not mean that al-Marri must be set free. Like others accused of terrorist activity in this country … al-Marri can be returned to civilian prosecutors, tried on criminal charges, and, if convicted, punished severely."