He gave U.S. officials their first in-depth look at the workings of terrorist training camps in Afghanistan and provided detailed information -- much of which still remains secret -- on more than 100 operatives of al Qaeda and other terrorist groups, according to documents his attorneys have submitted in advance of sentencing.
Ressam, for instance, named Abu Zubaida, then little known but later arrested in Pakistan, as a key aide to Osama bin Laden.
His attorneys say Ahmed Ressam was a valuable informant about al Qaeda and merits less prison time.
He may even have saved the lives of federal agents, his attorneys argue, by telling an FBI agent in May 2001 about a new trick of terrorist bombmakers: a paper detonator that can escape detection at airport security.
When Richard Reid, who tried to bring down an airliner with bombs hidden in his shoes, was arrested in Boston seven months later, the FBI agent who debriefed Ressam rushed there, recognized the paper detonator and may have saved the lives of investigators handling the shoes, according to defense documents.
Ressam did stop cooperating with the government, his attorneys concede, but only because "he has suffered memory deterioration" as a result of years of solitary confinement and "oppressive, repetitive interrogations."
Still, his attorneys say, Ressam is ready to cooperate again and his years of providing "immeasurable" information justify a "substantial reduction" in his sentence. They recommend 12 1/2 years. Coughenour has called Ressam's cooperation "startlingly helpful."
After his highly publicized arrest on Dec. 14, 1999, and before the attacks of Sept. 11, Ressam was the single most recognized "face" of terrorism in the United States, according to the Sept. 11 commission's report.
That report criticized federal officials for not taking full advantage of Ressam's knowledge before Sept. 11.
After the attacks on the World Trade Center and the Pentagon, Ressam was shown a group of photos. He singled out Zacarias Moussaoui, who was arrested before the attacks and last week admitted that bin Laden personally asked him to fly an airplane into the White House. Ressam said he remembered Moussaoui from an Afghan training camp.
"The Ressam identification would have broken the logjam," the Sept. 11 commission wrote.
Defense lawyers in Seattle said in court documents that federal prosecutors, in return for Ressam's cooperation, had initially "left us with hope" that he could be sentenced to 20 years or less.
But shortly before Ressam was scheduled to fly to New York to testify in a terrorism trial, the government delivered a "take it or leave it" offer of at least 27 years in prison, in return for Ressam's cooperation, the defense lawyers wrote. This tougher offer, they said, apparently came from John D. Ashcroft, who was then attorney general.
Ressam signed the agreement, his lawyers said, "despite our belief that we had been manipulated." But according to government prosecutors, Ressam, who was convicted of nine federal counts related to his planned airport attack, willingly agreed to cooperate, knowing that he could never receive a sentence of less than 27 years.
When he stopped cooperating in 2003, prosecutors said in court documents, he became subject to a "low end" sentence of 65 years in prison. Prosecutors say their request for 35 years is "extraordinarily generous."
"If Ressam had undergone a genuine change of heart, as opposed to merely trying to minimize his period of incarceration, he would continue to cooperate," the government said.
In addition to breaching his promise, the prosecutors said, Ressam's refusal to cooperate has undermined cases against two others accused in the plot to bomb the airport in Los Angeles: Abu Doha, arrested in England, and Samir Ait Mohamed, arrested in Canada.