U.S. officials say the FBI has arrested an American lawyer in connection with the deadly March 11 terrorist train bombings in Madrid, Spain.

Officials said that Brandon Mayfield was taken into custody Thursday in Portland, in the western U.S. state of Oregon after Spanish authorities reported finding what may be his fingerprints on a bag containing bomb-related materials. Agents also searched his home.

The 37-year-old man apparently was detained under a material witness warrant, which allows authorities to hold him indefinitely without charges while they carry out their investigation.

Law enforcement officials have declined to comment on the matter.

Mr. Mayfield, a former U.S. Army officer, converted to Islam in 1989. Authorities say he has been under around-the-clock FBI surveillance for some time.

The March 11 bombings of four Madrid commuter trains killed 191 people and wounded 2,000 others.

Spanish authorities suspect Moroccan Islamic extremists with possible connections to al-Qaida carried out the blasts in response to Spanish involvement in the war in Iraq.

Eighteen suspects are in Spanish jails in connection with the attacks.

Mr. Mayfield represented one of the chief defendants in the so-called "Portland Seven" terror case in a custody battle. Six members of a Portland terrorist cell pleaded guilty in October of plotting to fight with the Taliban against the United States in Afghanistan.