"The Internet is the key issue," Gilles Kepel, a prominent Arabist and a professor at the Institut d'Etudes Politiques, in Paris, told me recently. "It erases the frontiers between the dar al-lslam and the dar al-Kufr. It allows the propagation of a universal norm, with an Internet Sharia and fatwa system." Kepel was speaking of the Islamic legal code, which is administered by the clergy. Now one doesn't have to be in Saudi Arabia or Egypt to live under the rule of Islamic law. "Anyone can seek a ruling from his favorite sheikh in Mecca," Kepel said. "In the old days, one sought a fatwa from the sheikh who had the best knowledge. Now it is sought from the one with the best Web site."

To a large extent, Kepel argues, the Internet has replaced the Arabic satellite channels as a conduit of information and communication. "One can say that this war against the West started on television," he said, "but, for instance, with the decapitation of the poor hostages in Iraq and Saudi Arabia, those images were propagated via Webcams and the Internet. A jihadi subculture has been created that didn't exist before 9/11."

Because the Internet is anonymous, Islamist dissidents are less susceptible to government pressure. "There is no signature," Kepel said. "To some of us who have been trained as classicists, the cyber-world appears very much like the time before Gutenberg. Copyists used to add their own notes into a text, so you never know who was the real author."

Gabriel Weimann, a senior fellow at the United States Institute of Peace, has been monitoring terrorist Web sites for seven years. "When we started, there were only twelve sites," he told me. "Now there are more than four thousand." Every known terrorist group maintains more than one Web site, and often the sites are in different languages. "You can download music, videos, donate money, receive training," Weimann said. "It's a virtual training camp." There are two online magazines associated with Al Qaeda, Sawt al-Jihad (Voice of Jihad) and Muaskar al-Battar (Camp al-Battar), which feature how-to articles on kidnapping, poisoning, and murdering hostages. Specific targets, such as the Centers for Disease Control, in Atlanta, or FedWire, the money-clearing system operated by the Federal Reserve Board, are openly discussed. "We do see a rising focus on the U.S.," Weimann told me. "But some of this talk may be fake-a scare campaign."

One of the sites has been linked directly to terrorist acts. An editor of Sawt al-Jihad, Issa bin Saad al-Oshan, died in a gun battle with Saudi police on July 21, during a raid on a villa in Riyadh, where the head of Paul M. Johnson, Jr., the American hostage, was discovered in the freezer.

The importance of the Internet in the case of Madrid is disputed among experts. "Yes, the Internet has created a virtual ummah," Olivier Roy, an expert on political Islam at the French Centre National de la Recherche Scientifique, wrote to me recently. "The Web sites seem to attract the lonely Muslim cybernaut, who does remain in a virtual world. But Madrid's bombers used the Internet as a tool of communication. Their leaders had personal links with other Al Qaeda members, not virtual ones."

Thomas Hegghammer, the Norwegian investigator, divides the jihadi Internet community into three categories. "First, you have the message boards," he explained in a recent e-mail. "There you find the political and religious discussions among the sympathizers and potential recruits. The most important message boards for Al Qaeda sympathizers are Al Qal'ah (The Fortress), Al Sahat (The Fields), and Al Islah (Reform)." These boards, Hegghammer wrote, provide links to the "information hubs," where new radical-Islamist texts, declarations, and recordings are posted. "You often find these among the 'communities' at Yahoo, Lycos, and so on," Hegghammer continued. "There are many such sites, but the main one is Global Islamic Media." It was at this site that Hegghammer discovered the "Jihadi Iraq" document. "Finally, you have the 'mother sites,' which are run by people who get their material directly from the ideologues or operatives. They must not be confused with the myriad amateur sites (usually in English) set up by random sympathizers or bored kids."

Hegghammer pointed to several key sites associated with Al Qaeda, including Al Faruq (He Who Distinguishes Truth from Falsehood) and Markaz al-Dirasat wal-Buhuth al-Islamiyyah (Center for Islamic Study and Research). "Al Faruq is difficult to place geographically and organizationally, but it seems closer to the Afghanistan-based elements of Al Qaeda," Hegghammer wrote. Markaz al-Dirasat concentrates on Saudi Arabia. These sites move continuously, Hegghammer wrote, sometimes several times a day, to avoid being hacked by intelligence agencies or freelance Internet vigilantes. One of Al Qaeda's first sites, Al Neda, was operating until July 2002, when it was captured by an American who operates pornography sites. The Internet jihadis now cover themselves by stealing unguarded server space. Jihad videos have recently been discovered on servers belonging to George Washington University and the Arkansas Department of Highways and Transportation.

Last March, in Pakistan,Jamal Ismail, a reporter for Abu Dhabi TV showed me how he monitors the Al Faruq site. Each day, he receives an e-mail with a link, which leads him to the new address. Like several other jihadi sites, the Al Faruq site announces itself with a white stallion racing across the screen, which is the Al Qaeda logo. "Every few days, it announces a new name, but it is the same Web site with a new look," he told me. "It concentrates on Iraq, Saudi Arabia, and Afghanistan." In mid-July, I asked Ismail via e-mail if there was any discussion of the upcoming American Presidential election; the Department of Homeland Security had just announced contingency plans to postpone the election in the event that Al Qaeda attempts to disrupt it. "There is no new article like the Spanish one, but we are all expecting people to talk about it," Ismail said. Sageman said that he had seen "vague statements along the lines of 'We'll do to the U.S. the same as we did to Spain,'" but nothing specific or authoritative.

I went to Yahoo Groups and typed in "jihad." There were a hundred and ninety-two chat groups registered under that category. With my Arabic-speaking assistant, Nidal Daraiseh, I checked out qa13ah.net, which had 7,939 members. On March 12, the day after the train bombings, a message titled "The Goals of Al Qaeda in Attacking Madrid" had been posted by a writer calling himself Gallant Warrior. Echoing a theme that is frequently repeated on these sites, the writer noted that by carrying out its threat to Spain, Al Qaeda proved that its words were matched by actions: "Al Qaeda has sent a message to the crusading people: do not think that death and fear are only for the weak Muslims… Aznar, the American tail, has lost. And great fear has spread among the people of the countries in alliance with America. They will all be vanquished. Thank God for letting us live this long to see the jihad battalions in Europe. If anyone had predicted this three years ago, one would have said he was dreaming."

Another site I visited, ikhwan.net, was unusual in having a number of female correspondents. A writer named Murad chastised those who condemned the Madrid bombings. "You pity the deaths of those non-Muslims so quickly! If Muslims had died in their lands in the manner the writer discusses, would he have cried for them?" A woman named Bint al-Dawa responded, "Brother Murad, Islam does not allow the killing of innocent people." A man who called himself "Salahuddeen2" entered the discussion: "We have said that we are against the killing of civilians anywhere, but the enemies of God kill Muslim civilians every day and do not feel shame. They should drink from the same bitter cup."