A lawyer and former Army officer who converted to Islam was arrested as a material witness in the deadly train bombings in Spain, federal authorities said.

Brandon Mayfield was taken into custody Thursday by FBI agents, who also searched his home in the Portland suburb of Aloha.

It was the first known arrest in the United States with connections to the March 11 terrorist attacks in Madrid that killed 191 people and injured 2,000 others.

Mayfield, 37, was arrested on a material witness warrant and has not been charged with any crime, according to a senior law enforcement official in Washington D.C., speaking on condition of anonymity. A material witness warrant allows the government to hold people suspected of having direct knowledge about a crime or to allow time for further investigation into the witness.

Mayfield's fingerprints were found on materials related to the Madrid bombings (search), said a second senior law enforcement official, also speaking on condition of anonymity.

Beth Anne Steele, a spokeswoman for the FBI in Portland, confirmed two search warrants had been served Thursday in Washington County, which includes Aloha. She would not release further details.

Mayfield passed the Oregon bar in 2000 and largely kept a low profile in the Portland legal community until 2002, when he volunteered to represent Muslim terrorism suspect Jeffrey Battle (search) in a child custody case.

Battle was among six Portland area residents who were sentenced last year on charges of conspiring to wage war against the United States by helping Al Qaeda (search) and the former Taliban (search) rulers of Afghanistan.

Mayfield was not involved with Battle's defense in that case. Law enforcement officials in Washington did not know of any contacts between Mayfield and the other Portland terrorism defendants.

Mayfield had converted to Islam in the late 1980s and regularly attended Friday prayers at a Beaverton mosque, said mosque administrator Shahriar Ahmed.

Friends and family said they were shocked to learn of the arrest.

Outside their home near Portland late Thursday, Mona Mayfield described her husband as "a good man, a good father, a good husband." The couple have two sons, ages 10 and 15, and a 12-year-old daughter.

Portland attorney Tom Nelson, who described himself as a mentor, said he received a call Thursday afternoon from 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," Nelson told reporters outside Mayfield's home. "He is a regular, run-of-the-mill guy."

Nelson also said Mayfield had never traveled to Spain.

"Obviously, the government holds all the cards in these kinds of situations," Nelson told ABC's "Good Morning America" Friday. "It can release any kind of information it thinks it wants to release and the other side is prohibited to speaking on the merits so I can't speak to the merits."

"He's in no position, to, say, do forensic tests of his own" on the alleged fingerprints, Nelson added. He said he was speaking as a friend and was not acting as Mayfield's legal counsel.

Mayfield's father, Bill Mayfield, of Halstead, Kan., said his son spent most of his youth in Kansas and attended Washburn University in Topeka, Kan. Bill Mayfield said he was not able to talk to his son after the arrest.

"It's ludicrous. There's no way he did this," Bill Mayfield said.

Spanish authorities blame the attack on Islamic extremists, possibly linked to Al Qaida. Eighteen people have been charged to date, six with mass murder and the others with collaboration or with belonging to a terrorist organization.