Brandon Mayfield's fingerprints were found on a bag containing detonators like those used in the March 11 attack, the Spanish government said Friday.
The plastic shopping bag was found inside a stolen van left near the train station from which three of the four bombed trains departed, an Interior Ministry official said on condition of anonymity.
Mayfield, 37, a lawyer and former U.S. Army officer, was taken into custody Thursday by FBI agents, who also searched his home in the Portland suburb of Aloha.
"They went through every drawer, every piece of underwear, every book, every everything," family friend Tom Nelson told CBS News.
It was the first known arrest in the United States with connections to the March 11 terrorist attacks, which killed 191 people and injured 2,000 others.
"I'm upset that this has leaked out," said his wife, Mona Mayfield. "There has been no charges against my husband."
Brandon Mayfield was arrested on a material witness warrant and has not been charged with any crime.
A material witness warrant allows the Justice Department wide latitude to hold him until he tells what he knows to a grand jury or until he's charged, reports CBS News Correspondent Stephanie Lambidakis in Washington.
The detonators in the bag were of the same kind used in the attacks, the government has said previously.
The van was found in Alcala de Henares, about 20 miles northeast of Madrid. Inside, police also found an Arabic-language cassette tape with verses from the Quran, investigators have said.
The government's announcement of the van's discovery, after insisting that Basque separatists were the prime suspects, was its first public admission of a possible Islamic link in Spain's worst terrorist attack.
The bombings have been blamed on an alleged Moroccan-based Islamic extremist cell with possible links to al Qaeda.
Mayfield's family denies any connection between him and the bombings, the worst terrorist attack since Sept. 11, 2001, reports CBS News Correspondent John Blackstone. A friend said Mayfield never traveled to Spain.
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 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 and the former Taliban rulers of Afghanistan.
Mayfield, who grew up in Halstead, Kan., 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, who served in the U.S. Army, converted to Islam in recent years. Members of his mosque in Beaverton are trying to stay optimistic.
"I'm sure everything is fine. This has to be a mistake," mosque administrator Shariar Ahmed told CBS News.
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.
Nelson, an attorney, 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."
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 Qaeda. Eighteen people have been charged to date, six with mass murder and the others with collaboration or with belonging to a terrorist organization.