In Spain, Osman Ahmed has been a suspect for the Madrid bombings since April 4. He had been identified by people living near a decrepit rural cottage where the bombs used in the attack were assembled, Spanish court officials said. Fingerprints of several key suspects were found in the cottage.
http://www.guardian.co.uk/worldlatest/story/0,1280,-4182606,00.html

He was in Spain in 2003, but left months before the March 11 attacks, which were planted on four commuter trains and also injured more than 2,000 people.

The suspected terrorists' link to al-Qaida, Osama bin Laden's network, was not clear. Pisanu said the arrests in Italy ``blocked a dangerous terrorist group gravitating around al-Qaida.'' Officials in Brussels said that while there was insufficient evidence that the link existed, the possibility could not be ruled out.