The FBI on Thursday arrested Brandon Mayfield, a former army officer who recently volunteered to represent a terrorist suspect in a child custody case, and searched his home in the Portland suburb of Aloha.
A senior detective claimed that Mr Mayfield's fingerprints were found on materials related to the Madrid bombings, which killed 191 people and injured 2,000 others. Eighteen people have been charged so far in Spain - six with mass murder and the others with collaborating with or belonging to a terrorist organisation.
Mr Mayfield has been arrested on a material witness warrant which allows the government to hold those who are suspected of having direct knowledge of a crime, and gives the state time to carry out further investigations.
The use of these warrants by the justice department, which has detained dozens of people it claims have information relevant to an inquiry, are becoming increasingly controversial.
On Thursday night his family said they did not know where he had been taken.
Mr Mayfield converted to Islam in the late 1980s and qualified for the Oregon bar in 2000. He kept a low profile in the legal community until he volunteered to represent Jeffrey Battle in a child custody case. Battle was among six Portland-area residents sentenced last year for conspiring to wage war against the US by helping al-Qaida.
In the event, Mr Mayfield did not ultimately get involved in the child custody case.
Friends and family said they were shocked to learn of his arrest. Outside their home near Portland, Mona Mayfield described her husband as "a good man, a good father, a good husband."
"I know he is innocent," she told the north-west cable television news channel KGW.
The couple have two sons, 10 and 15, and a daughter, 12.
A Portland attorney, Tom Nelson, who described himself as a mentor, said he received a call on Thursday afternoon from Mr Mayfield asking for help.
"His wife was in tears because of the way the search was conducted. The FBI apparently hurt things in the house, left things in disarray," said Mr Nelson. "He is a regular run-of-the-mill guy." He said Mr Mayfield had never travelled to Spain.
Neighbours described the Mayfield family members, who have lived in Aloha for about 2 years, as courteous and hard-working. One said that Mr Mayfield was outside, tending his tomato patch, on Thursday morning before the FBI arrived.
"He's just a hard-working American man, father and husband, as far as I know," said Roy Witt, who lives next door.
Mr Mayfield's brother Kent said Brandon was a vocal critic of US foreign policy, but no terrorist.
"I think the reason they are holding him is because he is of the Muslim faith, and because he is not super happy with the Bush administration," he said. "So if that's a crime, well, you can burn half of us."