Stanley Cohen, a New York civil rights attorney who took the case of Sheik Mohamed Abdirahman Kariye, said the FBI tests showed the bags were free of explosives.
Cohen said he is negotiating with prosecutors to release Kariye pending trial.
"I'm talking to the government about a bail package," Cohen said. If that doesn't happen, he said, the defense would request another detention hearing.
Kariye, who served as an imam at the Islamic Center in Portland, was arrested Sept. 8 at Portland International Airport as he tried to board a flight to the United Arab Emirates.
He was charged with Social Security fraud dating from 1983 to 1995 and has pleaded not guilty. But at a detention hearing Sept. 10, assistant U.S. attorney Charles Gorder cited the tests in arguing Kariye should be held in jail until trial.
The case is one of scores around the country of Muslim men held on charges unrelated to terrorism, although they were arrested by federal agents or members of a terrorism task force.
Cohen, a New York-based civil rights lawyer who took Kariye's case over the weekend, said the FBI sent a letter explaining the negative test results to U.S. District Judge Donald Ashmanskas on Wednesday.
FBI spokesman Beth Anne Steele declined to discuss the test results Sunday and referred questions to the U.S. Attorney's Office. Phone messages left there were not returned.
At a Sept. 10 detention hearing, Robert Ramos, a senior inspector for the U.S. Customs Service, testified that two bags carried by Kariye's party tested positive for explosives residue.
Ramos added that he had tested hundreds of bags, and Kariye's was the first to test positive for explosives.
Residue collected from a third bag tested positive for cocaine, but a second test on the bag turned up negative for cocaine, Ramos testified.
Cohen said he may argue the residue test used at the airport is faulty.
Cohen spoke about Kariye's case Saturday at a gathering of about 200 Muslims, many in skull caps and sporting bushy beards. To cheers and applause, he advised the Portland crowd not to cooperate with law enforcement in the terror probe unless they have a lawyer present.
He urged them to fight back "in an appropriate and lawful manner."
Police have not said they suspect Kariye of terror-related offenses, other than to say that he was detained by the FBI-led Joint Terrorism Task Force. Kariye's supporters question why the task force was even involved, because Kariye has only been charged with Social Security fraud.
Searches of public records show Kariye was affiliated in 1992 with a Chicago-area Muslim charity, Global Relief Foundation, that was later investigated for ties to Al Qaeda. A lawyer for the charity denied it was used to funnel money to terrorism, and said Kariye's name does not appear on charity documents after 1992.
By Andrew Kramer