Yahoo! News - Family of Terror Suspect Waits for Answers
"It was a move that raised red flags for counterterrorism expert Michael Greenberger, a former U.S. Department of Justice official.
'The fact that they are using the material witness statute shows they don't have probable cause to arrest him,' said Greenberger, who now heads the University of Maryland's Center for Health and Homeland Security. 'They're scrambling to find a way to detain him.'"
Brandon Mayfield, an Oregon resident and convert to Islam, has been detained by the FBI as a material witness. There was a partial match of his fingerprints to a print on a detonator related to the bombing in Spain.
The Christian Science Monitor ran a good story on the use of material witness statute almost two years ago.