With Spanish, British and Syrian interrogators lining up with requests to question him, he has turned out to be a prize catch, a man who is not a bombmaker or operational planner but one of the jihad movement's prime theorists for the post-Sept. 11, 2001, world.

Counterterrorism officials and analysts see Nasar's theories in action in major terrorist attacks in Casablanca in 2003, Madrid in 2004 and London in 2005. In each case, the perpetrators organized themselves into local, self-sustaining cells that acted on their own but also likely accepted guidance from visiting emissaries of the global movement.