Brandon Mayfield, a U.S. citizen, was taken into custody on a material witness warrant, said a senior law enforcement official in Washington D.C., speaking on condition of anonymity. The arrest is the first known in the United States with connections to the March 11 bombings in Madrid.
The FBI also searched the man's home, which he shares with his wife, the official said.
Mayfield's fingerprints were found on materials related to the Madrid bombings, said a second senior law enforcement official, also speaking on condition of anonymity.
Outside Mayfield's suburban Portland home Thursday evening, his wife, Mona, said, "As you can imagine, today has been quite dramatic. My husband is a good man, a good father, a good husband. It's just unfair. It's unfair to myself and it is unfair to my children."
She said her husband was 37, and that the two have three children, two boys ages 10 and 15, and a girl, aged 12. Her husband was born in the small Oregon coastal community of Coos Bay, she said, and that he is a former Army officer.
His undergraduate degree is from Portland State University, Mona Mayfield said, and his law degree is from Lewis & Clark University in Portland. He converted to Islam in 1989, she said, and attends a mosque in nearby Beaverton, which was reportedly searched by FBI agents Thursday.
Portland attorney Tom Nelson, who described himself as Mayfield's friend and mentor, said Thursday afternoon that he received a call from Mayfield Thursday, pleading for help. Nelson said Mayfield would be represented by a public defender.
"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.
Mayfield passed the Oregon bar in 2000, according to the Oregon Bar Association.
Material witness warrants, usually kept confidential by a federal judge, are used by the government to hold people suspected of having direct knowledge about a crime or to allow time for further investigation into the witness. Suspects may be held indefinitely without formal charges.
Officials would not provide any further details about the man or his alleged connection with the Madrid bombings, which killed 191 people and injured 2,000 others. Spanish authorities blame the attack on Islamic extremists, possibly linked to al Qaeda.
Eighteen people have been charged to date in Spain — six charged with mass murder and the others with collaboration or with belonging to a terrorist organization. The FBI and other U.S. agencies have warned that al Qaeda or its sympathizers might attempt to attack mass transit systems in major U.S. cities this summer.
Earlier this year, in Portland, the last of six men and a woman were sentenced on charges of conspiring to wage war against the United States by helping al Qaeda and the former Taliban rulers of Afghanistan.
Mayfield represented one of those people, Jeffrey Leon Battle, in a custody case involving Battle's son. Law enforcement officials did not know of any further contacts between Mayfield and any of the other Portland terrorism defendants.
Mayfield had attempted to have Battle's son, who went by the Muslim name Esau in Portland, placed in the custody of an uncle who had also converted to Islam, rather than with his biological mother and Battle's former wife Angela Rowden of Houston. Rowden was awarded custody of the boy, who now goes by the name Geoffrey.