FBI agents have arrested a lawyer from the US state of Oregon in connection with the Madrid train bombings which killed 191 people.
Mr Mayfield is being held as a material witness in the case
Muslim-convert Brandon Mayfield, from Portland, is the first known person in the US to be held over the attacks.
Newsweek magazine reported that the man's fingerprints had been found on a bag that had carried explosives used in the attacks.
Spain has brought provisional charges against more than a dozen suspects.
The 11 March bombings, Spain's worst terrorist attacks, are believed to have been the work of Islamic militants with links to al-Qaeda.
Mr Mayfield, 37, is being held as a material witness in the case, but there was no officials statement on his alleged connection with the bombings.
Newsweek said the lawyer had done child custody work for one of six Oregon
Muslims convicted last year of trying to travel to Afghanistan to help al-Qaeda.
Reports say authorities searched Mr Mayfield's home and office.
Mr Mayfield's brother told Reuters news agency his brother had "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," he said.
"So if that's a crime, well you can burn half of us."