In the eyes of many terrorism experts, Nicholas Berg's tragic beheading has elevated Abu Musab al-Zarqawi to a level approaching that of Osama bin Laden. The comparison seems appropriate, as Zarqawi, like bin Laden, has surrounded himself with other powerful extremists and several valuable contacts in the jihad underworld.

Bin Laden chose a well-connected Egyptian, Ayman al-Zawahiri, as his deputy in order to take advantage of Zawahiri's widespread network of terrorists and political knowledge. By the same token, Zarqawi appears to be teaming up with Mustafa Setmariam Nasar, a man who, so far, has not attracted much attention, but whose influence on Zarqawi cannot be overlooked. According to Italian military intelligence, both Zarqawi and Nasar are currently in Iraq, masterminding attacks there and throughout the world.

Nasar, better known by his nome de guerre, Abu Musab al-Suri, is well known to Spanish authorities, who wrote extensively about him in the September 2003 indictment of the Madrid al Qaeda cell. A Syrian veteran of the Afghan war against the Soviets, he spent several years in Madrid in the mid-1990s and acquired Spanish citizenship by marrying a Spanish convert. While in Spain, he befriended a fellow Syrian, Imad Eddin Yarkas, the leader of the Madrid cell. Both had fled Syria in the beginning of the 1980s after the Syrian regime violently cracked down on the Muslim Brotherhood.

But while Yarkas immediately went to Spain, Suri went to Afghanistan, where he met bin Laden and Abdullah Azzam, bin Laden's mentor and one of the founders of al Qaeda. According to the Spanish indictment of the Madrid cell, when al Qaeda moved to Sudan in 1991, Suri remained in Afghanistan, traveling sporadically to Khartoum to meet with bin Laden. In 1995 he moved to Europe and lived between Madrid and London, where he was one of the leading minds of the local Islamist scene now referred to as "Londonistan."

But upon realizing that British authorities suspected his involvement in the 1995 Paris Metro bombings, Suri decided to move back to Afghanistan, where he ran a terrorist training camp. According to Spanish authorities, while in Afghanistan Suri maintained contacts with both Osama bin Laden and former Taliban leader Mullah Omar. Suri also became Emir of the Syrians associated with al Qaeda, a title that proves his importance in the organization. The man who facilitated Suri's move to Afghanistan was another Syrian national, Mohamed Bahaiah, whom Spanish authorities have described as "bin Laden's courier in Europe" and who used to travel in Spain extensively.

Bahaiah is just one of Suri's contacts in Spain. Suri maintained a network of operatives there and he is now believed to have masterminded the March 11 train bombings in Madrid. Information provided by some of the individuals detained in relation with the attacks has shown that an intermediary of Suri traveled to Spain at the end of 2003. According to police inquiries, after arriving in Spain, the intermediary made contact with one of the bombers and passed on to him instructions from Suri. After staying in Spain for a few days, the messenger left for London, where Suri had instructed him to activate sleeper cells. Interpol has issued an arrest warrant for Suri in relation with the Madrid bombings. If this information is confirmed, it would be definitive proof that the Zarqawi network was behind the Madrid attacks.

But aside from his terrorist contacts throughout the world, Suri's importance lies in the profound political and religious influence he wields over Zarqawi. While Zarqawi is a high school dropout with scant knowledge of world affairs, Suri has a long history of writing about politics and is a Koranic expert who boasts a large following in the radical Islamic underworld. During his stay in London, Suri was one of the editors of the ultra-radical Al Ansar magazine, which for years published propaganda from dozens of Islamic terrorist groups. On one instance, Al Ansar published a fatwah that justified the killing of children and women in Algeria by the Algerian terrorist group GIA. The fatwah was issued by the Palestinian cleric Abu Qatada, a man that Spanish authorities have described as "Bin Laden's ambassador to Europe" and who also served as editor-in-chief of Al Ansar.

A further glance at Suri's extremist ideology is provided by tapes of his sermons that were seized in the apartment of a member of an Algerian terrorist cell dismantled by Italian authorities in Naples in 2000. The tapes reveal Suri's deep hatred for Shiites, whom he considers deviators from pure Islam. While other al Qaeda leaders have expressed their contempt for Shiites but nonetheless cooperated with Shiite groups, Suri categorically rejects any form of cooperation between Sunnis and Shiites. In fact, he points at the "negative influence" that Shiite groups have had on the Palestinian struggle, as some groups like Hamas have decided to work with Shiite groups like Hezbollah. This same contempt for Shiites can be seen in a letter written by Zarqawi last February, in which he openly incited a sectarian war in Iraq between Sunnis and Shiites. Zarqawi urged his followers to carry out attacks against Shiites because they "have declared a subtle war against Islam."

The letter reveals the influence that the older and better-educated Suri has on Zarqawi, who did not express any anti-Shiite sentiment while working closely with the Iranian government (as revealed by the confessions of Shadi Abdallah, a terrorist linked to Zarqawi who was arrested in Germany). Just as Zawahiri's ideas influenced bin Laden's actions and worldview, then, it appears that Zarqawi is acting in accordance with Suri's views. It is too early to say whether--with bin Laden and Zawahiri reportedly relegated to Waziristan--Zarqawi and Suri have become the world's most dangerous terrorist duo. It's also too early to say whether Iraq will represent for them what Afghanistan represented for the other duo. But it is clear that the two have managed to operate undetected for almost a decade and are now reaping the fruits of their work. It is now extremely important to understand Suri's ideas in order to penetrate Zarqawi's violent mindset.

--Lorenzo Vidino is a senior terrorism analyst at the Investigative Project, a Washington-based counterterrorism institute.