Madrid, Spain ? 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.

Two senior U.S. law enforcement officials in Washington said Brandon Mayfield, 37, had been under FBI surveillance for several weeks after a single fingerprint was found.

Mayfield, a 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.

The U.S. officials, who spoke on condition of anonymity, said agents had intended to continue the surveillance longer, but when it became clear that news about him might leak out, the Justice Department placed him in custody.

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

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

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.

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