An FBI spokeswoman refused to confirm or deny the detention on Thursday, saying only that two search warrants had been issued in connection with an unspecified investigation.
Newsweek said the lawyer, whom it named as Brandon Mayfield, had done child custody work for one of six Oregon Muslims convicted last year of trying to travel to Afghanistan
to help al-Qaida.
The magazine said the man's fingerprints were found on bomb-related evidence associated with the Madrid attacks.
The magazine quoted sources as saying the lawyer had been under round-the-clock surveillance by the FBI for some time and that he was picked up by FBI agents in Portland on Thursday.
Mayfield's brother Kent, reached at his home in Halstead, Kansas, confirmed the arrest and the surveillance and said he suspected government agents had tapped his telephone line.
"They yanked him out of his home without giving any kind of information to his family," Kent Mayfield told Reuters. "I can swear up and down my brother has no connection to terrorist attacks."
"I think the reason they are holding him is because he is of the Muslim faith and because he is not super happy with the Bush administration. So if that's a crime, well you can burn half of us"
Brandon Mayfield, brother
Newsweek said Mayfield was being held as a "material witness" in a grand jury investigation and that allowed the Justice Department to hold him in secret without formally filing charges against him. A Justice Department spokesman declined comment.
Brandon Mayfield is a former Army officer and a vocal critic of the Bush Administration's foreign policy, Kent Mayfield said, but he declined to elaborate.
Agents raided Brandon Mayfield's home, taking computer equipment and his wife's credit cards, leaving her with just a driver's license, Kent Mayfield said, adding that the couple had three children.
"I think the reason they are holding him is because he is of the Muslim faith and because he is not super happy with the Bush administration. So if that's a crime, well you can burn half of us," Kent Mayfield said.