"Material-witness warrants are used by the government to hold people suspected of having direct knowledge about a crime or to give agents more time to investigate." http://www.latimes.com/news/nationworld/world/la-na-madrid7may07,1,303922.st... ALOHA, Ore.  A Portland lawyer was detained Thursday by the FBI after federal officials linked his fingerprint to bomb-related evidence associated with the Madrid railway attacks that killed 191 people in March, a federal law enforcement official said. The arrest of Brandon Mayfield, 37, raises the possibility of a U.S. connection to the March 11 bombings, which Spanish authorities have blamed on Islamic extremists. The former Army officer, a Muslim convert, was held on a material-witness warrant after the FBI searched his home in the Portland suburb of Aloha. Mayfield's arrest was first reported Thursday on Newsweek magazine's website. He has not been charged with any crime, and the federal official stressed that the investigation was continuing and is in many ways preliminary. Spanish officials cautioned earlier this week that they did not consider the fingerprint evidence to be conclusive. Material-witness warrants are used by the government to hold people suspected of having direct knowledge about a crime or to give agents more time to investigate. <snip>