But as Ressam, 37, awaits sentencing Wednesday, prosecutors say he could have done more.
The government is seeking 35 years behind bars for Ressam. Prosecutors say he has stopped talking with investigators and consequently has jeopardized two related terrorism prosecutions.
Federal agents boosted security at the downtown U.S. District Court on Wednesday in advance of the hearing. Police buses were used to block off streets around the courthouse.
Ressam's public defenders are asking for 12½ years — and say Ressam is willing to continue cooperating but doesn't remember as much as he used to.
Ressam was arrested in Port Angeles in December 1999 as he drove off a ferry from British Columbia. He ran when Customs agents searched his car but was caught about six blocks away. In his trunk were explosives more powerful than TNT and digital watches that could be used as timers, authorities said.
Because Ressam had a one-night reservation for a motel just blocks from the Space Needle in Seattle, the mayor called off most of the city's celebrations to welcome the year 2000.
Also found in the car, authorities said, was a California tour book with Ressam's fingerprints on pages showing downtown Los Angeles and the Transamerica tower in San Francisco. In Ressam's apartment was a map with circles around Los Angeles International Airport, and two other California airports.
The Algerian national was convicted in April 2001 of nine charges, including smuggling and terrorist conspiracy, in what investigators described as an unsuccessful plot to bomb Los Angeles International Airport on Jan. 1, 2000.
Facing up to 130 years in prison, he began to talk, and prosecutors and defense lawyers agree he provided extensive information about terror camps in Afghanistan. But in 2003, defense lawyers called off further talks, saying he his mental state had deteriorated because of long periods of solitary confinement.
His sentencing has been repeatedly delayed to ensure his cooperation in other cases.
Prosecutors now say that without his continued help, they may have to drop terrorism charges against two other suspects in the bomb plot. Abu Doha and Samir Ait Mohamed are awaiting extradition to the United States — Doha in Britain, Mohamed in Canada.
Ressam's testimony helped convict Mokhtar Haouari of supplying fake identification and cash for the millennium bomb plot. Haouari was sentenced in New York to 24 years in prison.
In December 2002, Ressam met with German justice officials who questioned him about al Qaeda for the trial of a Moroccan charged with supporting the Sept. 11, 2001, terrorist hijackers. Mounir el Motassadeq was convicted in February 2003 and sentenced to 15 years in prison.