Juan Aviles, a history professor at Madrid's Autonomous University and an adviser to the Civil Guard, told me, "From our Western point of view, it doesn't make sense that the killings of Atocha are meaningless. In Spain, we expect ETA to behave in certain ways. With Al Qaeda, the real dimensions of the threat are not known. And that produces uneasiness."

In the weeks after the March 11 attacks, Spanish police combed the immigrant neighborhoods outside Madrid, carrying photographs of suspects. "We didn't have them perfectly located, but we knew they were in Leganes," a police official told me. Leganes is a bland suburb of five-story red brick apartment complexes. The wide streets are lined with evenly spaced, adolescent oaks. In the mornings, the sidewalks are full of commuters rushing for the trains; then the place is vacant, except for grandmothers and strollers. In the evenings, the commuters return and close their doors.

At three o'clock on the afternoon of April 3, the day after the discovery of the bomb on the AVE tracks, police approached an apartment building on Calle Carmen Martin Gaite. They saw a young Moroccan man with a baseball cap on backward who was taking out trash. He yelled something in Arabic, then ran away at an impressive pace. (He turned out to be a track champion; the police did not catch him, and he remains at large.) A moment later, voices cried out, "Allabu Akhbar," and machine-gun fire from the second floor of the apartment house raked the street, scattering the cops. Over the next few hours, the police tactical unit, Grupo Especial de Operaciones, evacuated the residents of nearby apartments. Tanks and helicopters moved in, and the siege of Leganes began.

Inside the apartment were seven young men. Most of them were Moroccan immigrants who had come to Europe seeking economic opportunity. They had gone through a period of becoming "Westernized"-that is to say, they had been drinkers, drug dealers, womanizers. They hung out in cybercafes. They folded into the ethnic mix of urban Madrid. But they also lived in the European underground of Islamic radicalism, whose members were recruited more often in prison than in the training camps of Afghanistan.

Their leader was Sarhane Ben Abdelmajid Fakhet, who was thirty-five years old and had a round, fleshy face and a patchy beard. He was a real-estate agent who had come to Madrid eight years earlier on a scholarship to study economics. His boss told the Spanish press that Fakhet was "a wonderful salesman," who held the record for the number of apartments sold in a month. Yet he did not talk to his coworkers or make friends with other Spaniards; he remained sequestered in his Muslim world.

"He was very soft and well educated," Moneir Mahmoud Aly el-Messery, the imam at the principal mosque in Madrid, told me. The mosque-a massive marble structure, built with Saudi money-is the center of Muslim cultural life in the Spanish capital. It overlooks the M-30, one of the main freeways feeding into Madrid. When Fakhet was a student, he worked in the restaurant that is attached to the mosque, and he sometimes came to Messery's weekly religion class. In the beginning, the imam noticed that

Fakhet spoke familiarly to women as well as to men. "Then, for three or four years, I sensed that he had some extremist thoughts," Messery recalled. After class, Fakhet would ask telling questions, such as whether the imam believed that the leaders of the Arab countries were true believers, or if Islam authorized the use of force to spread the religion. Last year, he married a sixteen-year-old Moroccan girl who veiled her face and dressed entirely in black, including gloves. His performance at work declined, and he eventually stopped showing up altogether. According to police, he attended meetings with a small group of fellow Muslims at a barbershop in Madrid, where the men would drink holy water from Mecca. Police believe that this ritual was aimed at absolving the men of the sin of suicide, which is condemned by Islam.

Soon after the attacks of September 11, the imam had a dream about Fakhet. "Sarhane was in his kitchen, cooking on the stove," he recalled. "I saw what he was cooking was a big pot of worms. He tried to give me a plate of the food to eat. I said no. I said, 'Please clean the kitchen!' " Days later, the imam confronted Fakhet. "This is a message from God!" the imam said to him. "The kitchen is the thought, and the thought is dirty." Fakhet didn't respond. "He's a very cold person," the imam told me.

Fakhet was not the only young man in the M-30 mosque who had taken a turn toward extremism. Amer Azizi, a thirty-six-year-old Moroccan who was a veteran of jihad in Bosnia and Afghanistan, had been indicted in Spain for helping to plan the September 11 attacks. (He was accused of setting up the July 2001 meeting between Atta and other conspirators in Salou.) Among people who frequented the mosque, Azizi had the reputation of being a drug addict, although he attended some classes on Islam along with Fakhet. In June 2000, when the Arab countries' ambassadors to Spain came to the mosque to mourn the death of the Syrian dictator Hafez al-Assad, Azizi insulted them, yelling, "Why do you come to pray for an infidel?" Police charge him with being a senior member of Al Qaeda and the leader of the Moroccan Islamic Combat Group, which was responsible for five bombings in Casablanca in May 2003. He fled Spain just before his indictment.

Another of Fakhet's friends was Jamal Ahmidan, a drug dealer who police say financed the March 11 bombings with seventy pounds of hashish. Messery blamed an Islamist cleric in London,Abu Qatada, a radical Palestinian from Jordan who emigrated to Britain as a refugee in 1994. After September 11, police in Hamburg found eighteen tapes of Abu Qatada's sermons in Mohamed Atta's apartment there. British authorities arrested him in October 2002, but he still wields great authority among Islamists around the world. The imam told me, "It was as if there were black hands behind a curtain pushing these young men."

At six o'clock in the evening on April 3, three hours after the start of the Leganes siege, a handwritten fax in Arabic, signed by Abu Dujan al-Afghani, arrived at ABC, a conservative daily in Madrid. Referring to the bomb found beside the AVE tracks the day before, the author argues that it failed to explode because "our objective was only to warn you and show you that we have the power and capacity, with the permission of Allah, to attack you when and how we want." The letter demanded that Spain withdraw its troops from both Iraq and Afghanistan by the following Sunday. Otherwise, "we will turn Spain into an inferno and make your blood flow like rivers." On the surface, the fax represented another turn toward tactical political thinking; more likely, it was an attempt to salvage a bungled operation.

Outside the Leganes apartment, the police attempted to negotiate, but the cornered terrorists cried out, "We will die killing!" Phone calls that they made to relatives during the siege confirmed their intentions. They also attempted to call Abu Qatada in London's Belmarsh Prison, apparently seeking a fatwa that would morally sanction their suicide.

Instead of turning off the electricity and waiting them out, the police decided to storm the apartment. They ordered the terrorists to come out "naked and with your hands up." One of the occupants responded, "Come in and we'll talk." At 9:05 p.m., the police blew the lock on the door and fired tear gas into the room. Almost immediately, an explosion shattered the apartment, killing the terrorists and a police officer. The blast was so intense that it took days before the authorities could determine how many people had been in the apartment. The body of Jamal Ahmidan was hurled through the walls and into a swimming pool. One of the seven bodies still has not been identified.