The fingerprints of an American lawyer arrested in the Madrid terror bombing probe were found on a plastic shopping bag containing detonators like those used in the train attacks, the Spanish government said Friday.

The bag was found inside a stolen white van left near a train station that three of the four bombed trains departed from, an Interior Ministry official said.

Brandon Mayfield, a 37-year-old former Army officer who converted to Islam, was taken into custody Thursday by FBI agents, who also searched his home in the Portland, Ore., suburb of Aloha.

Mayfield had been under FBI surveillance for several weeks after the fingerprint discovery, two senior U.S. law enforcement officials told The Associated Press on condition of anonymity.

They said agents had intended to monitor him but when it became clear that news about him might leak out, the Justice Department decided to place him in custody.

It was the first known arrest of a U.S. citizen in connection with the March 11 commuter train attack (search) in Madrid that killed 191 people and injured 2,000 others.

The Spanish official declined to give more details and the National Court, where a judge is leading an investigation into the attacks, said it had no information on Mayfield.

The seven detonators found inside the bag were of the same kind used in the train bombings, the government says. Authorities also found an Arabic-language cassette tape with verses of the Quran inside the van, which was discovered in Alcala de Henares, about 20 miles northeast of Madrid, hours after the attacks.

The announcement of the van's discovery was the government's first public admission of a possible Islamic link in Spain's worst terrorist attack, which it initially blamed on Basque separatists.

Other fingerprints found inside the van included those of Moroccan Jamal Zougam (search), a prime suspect in the attacks who is under arrest and charged with mass murder and terrorism.

Police say cell phones used to activate the detonators in the backpack bombs used in the train attacks were traced to the cell phone store Zougam ran in Madrid.

Witnesses in Alcala de Henares reported seeing men wearing ski masks and carrying backpacks get out of the van on the morning of the attacks, headed for the train station.

The bombings have been blamed on an alleged Moroccan-based Islamic extremist cell with possible links to Al Qaeda (search). Eighteen people have been charged so far, six with mass murder and the others with collaboration or with belonging to a terrorist organization.

He was arrested Thursday on a material witness warrant and has not been charged with any crime, according to the senior officials in Washington. 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.

As a former Army officer, Mayfield's fingerprints would be on file with the government. A law enforcement official said the fingerprints were not on file because of any crime or as part of the government's terrorism databases.

Beth Anne Steele, a spokeswoman for the FBI in Portland, said 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 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-Qaida and the former Taliban 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 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 by 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 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 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.