This past week, when massive simultaneous explosions ripped through Spanish commuter trains in Madrid (killing 200), a search for possible culprits began immediately--with all eyes on the possibility of involvement by radical Islamic militant groups allied with al Qaeda. This search is ongoing, and Spanish authorities are far from making any definitive conclusions as to the identities of the bombing culprits. However, clues from the investigation seem to increasingly confirm that this attack was committed by supporters of Osama bin Laden and his cadre--not (necessarily) the persistent Basque separatist terror group known as ETA. The fact that Spanish minister of interiors, Angel Acebes, announced on Sunday that investigators will travel to Morocco to pursue leads makes it clear that the Islamic hypothesis is being seriously considered.

At first, Spanish authorities were quick to discount the bin Laden link, emphasizing the relatively strong circumstantial evidence pointing to ETA involvement. Some of this evidence is admittedly difficult to ignore. Last Christmas Eve, police arrested several ETA members in Madrid, supposedly preparing for a potential terror attack on the Madrid rail system. Moreover, less than three weeks ago, a van allegedly belonging to ETA was seized en route to Madrid carrying 66 pounds of Titadine and an additional 1,100 pounds of explosive. Titadine, a dynamite-like substance, has been used previously by ETA in its campaign of terrorism--and statements by authorities reveal that the Madrid attacks were caused by packs of Titadine reinforced with PETN or a plastic explosive, each device weighing around 20-25 pounds.

Of course, not everything led in the direction of the Basque extremists. Though the Titadine link seems compelling on its face, police added that the detonators used in the Madrid bombings were made of copper--not the typical aluminum ones favored by ETA. The group had never before attempted such a bloody and merciless operation; when it did target civilians, more often than not, there was a phoned-in warning first. Individuals claiming to represent ETA quickly contacted Basque and Spanish media sources to disclaim any role in the latest Madrid attacks.

Just as quickly, another notorious terrorist movement eagerly stepped forward to announce its responsibility in causing the deadly bombings: namely, al Qaeda. By the end of the week, Spanish authorities had received an unauthenticated e-mail from the "Abu Hafs Al-Masri Brigades" and a videotaped message from a previously unknown terrorist calling himself Abu Dujan Al-Afghani (and claiming to be al Qaeda's military chief in Europe). Osama bin Laden himself released an audiotape last October containing threats to "carry out martyrdom operations in and outside the United States" in "retaliation" against all nations who aided U.S. forces in Iraq. Bin Laden warned several particular countries--including Spain--that these operations would be carried out against them "at the proper time and place...until you stop being unjust." Perhaps the "proper time and place" was the eve of key government elections in Spain, amid heavy popular opposition to Spain's ongoing military role in Iraq.

But the strongest evidence linking the deadly Madrid bombings to al Qaeda might come from the identity of the men arrested on Saturday by Spanish police. In fact, two of the three Moroccans who investigators believe are linked to the mobile phone that was found in a bag with an unexploded bomb are known to have had ties to an al Qaeda cell dismantled in Madrid by Spanish authorities in the fall of 2001. One of them, Jamal Zougam, was described as a "follower" of Imad Eddine Barakat Yarkas, the leader of the Madrid cell--a man who kept close contacts with high-ranking al Qaeda operatives throughout the world and who is currently in jail, accused of having had a significant role in the planning of 9/11. The search of Zougam's Madrid apartment in July of 2001 led police to find the telephone numbers of many known supporters of Osama bin Laden and several radical Islamic books and tapes. One of these tapes, called "The Islamic Movement in the West," contained an interview with bin Laden. The name of another of the three arrested Moroccans, Mohamed Chaoui, was heard by Spanish intelligence while monitoring a phone conversation between Yarkas and another member of the cell known as Abdulak Al Magrebi. Al Magrebi was recorded by Spanish authorities telling Yarkas his intention to travel to Tangiers to establish a relationship with Chaoui "because he is related to Said Chedadi." Said Chedadi was an important member of al Qaeda's cell in Madrid, primarily involved in raising funds for Arab-Afghan mujahedeen in Chechnya. He traveled to London together with Yarkas to deliver the money to Palestinian cleric Abu Qutada--the man that, prior to his arrest in Great Britain, served as al Qaeda's spiritual leader in Europe.

While neither Zougam nor Chaoui appear to have been major players in the Madrid cell, their involvement in the cell's activities is well documented. The fact that Moroccan authorities have revealed that Zougam was put under surveillance after last May's Casablanca bombings because he was believed to be linked to the group responsible for the attacks (Salafia Jihadia) reinforces the suspect that Islamic fundamentalists are behind the Madrid attacks. Over the next days more, investigators will hopefully learn more details about the plot, but if Spanish authorities will confirm that Zougam and Chaoui have had a role in it, the suspicion that Madrid has been Al Qaeda's first major successful attack in Europe will definitely become stronger.

These events do not take place in a vacuum. The "trains of death" in Madrid come 911 days after the September 11 suicide hijackings, and less than a year following a series of jarring suicide bombings in nearby Casablanca. If al Qaeda is ultimately deemed responsible for Spain's "3/11" disaster, it will then have given proof of its ability to strike again in the heart of the West despite the vigorous intelligence efforts to dismantle its terrorist sleeper cell network. Moreover, it has managed to influence the outcome of the elections of a major European democracy, as Spanish voters decided to place the blame of the massacre on the ruling government's support for the U.S.-led war in Iraq.

--Lorenzo Vidino and Evan Kohlmann are terrorism analysts for the Washington, D.C.-based Investigative Project. Kohlmann is author of the upcoming book, Al Qaida's Jihad in Europe: the Afghan-Bosnian Network, to be released in June.