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Europe’s worst attack: Spanish prosecutors’ efforts to convict the accused Madrid bombers were hampered by the group’s lack of structure.

Ask the Author: Send questions for Marc Sageman to letters@ForeignPolicy.com by March 25, and we will post his answers on March 31 here.

When British police broke down Younis Tsouli’s door in October 2005 in a leafy west London neighborhood, they suspected the 22-year-old college student, the son of a Moroccan diplomat, of little more than having traded e-mails with men planning a bombing in Bosnia. It was only after they began examining the hard drive on Tsouli’s computer that they realized they had stumbled upon one of the most infamous—and unlikely—cyberjihadists in the world.

Tsouli’s online username, as they discovered, was Irhabi007 (“Terrorist007” in Arabic). It was a moniker well known to international counterterrorism officials. Since 2004, this young man, with no history of radical activity, had become one of the world’s most influential propagandists in jihadi chatrooms. It had been the online images of the war in Iraq that first radicalized him. He began spending his days creating and hacking dozens of Web sites in order to upload videos of beheadings and suicide bombings in Iraq and post links to the texts of...