Mohamed Merah, the 23-year-old Islamist gunman who hunted down three Jewish children and a rabbi after murdering three French paratroopers in Toulouse last month, didn't act alone. In his journey from the slums of Toulouse, to the local mosques, to the terrorist training camps in Afghanistan and Pakistan that he described to French police, to filming his murder of the terrified children in order to post video clips on the web, Mr. Merah was following a path marked out years earlier by the coldblooded jihadist theoretician Abu Musab al-Suri.

[SURI] European Pressphoto Agency

Abu Musab al-Suri, in an undated photo released by the U.S. government's Rewards for Justice program around 2004. He's been called 'the most dangerous terrorist you've never heard of.

What is perhaps more disturbing, Mr. al-Suri was recently set free from prison in Damascus, Syria, and his current whereabouts are unknown. Turned over to Syria after his capture by the CIA in late 2005, Mr. al-Suri was released sometime in December (according to intelligence sources and jihadist websites) by the regime of President Bashar al-Assad—a move apparently intended to warn the West of the consequences for opposing his rule.

Barely noticed in the midst of Mr. Assad's own brutal assaults on civilians, Mr. al-Suri's release may well contribute to the emergence of more attackers like Mr. Merah in the West. "His videos are already being reuploaded. His audios, reposted," wrote Jarret Brachman, a former CIA analyst and the former director of West Point's Center for Combating Terrorism, in a blog post after the news of Mr. al-Suri's release first appeared on jihadist sites.

Once called "the most dangerous terrorist you've never heard of" by CNN, Mr. al-Suri, whose real name is Mustafa Setmariam Nasar, served in the days before 9/11 as the facilitator who took Western reporters to meet with Osama bin Laden in Afghanistan. Photographs of him from those trips show a well-built man with pale white skin, a red beard and blue eyes who—Afghan garb aside—would not look out of place in an Irish pub or a cafe in Brussels.

[SURI2] Associated Press

Mr. al-Suri in a photo released by the U.S. government's Rewards for Justice program around 2005. Mr. al-Suri scolded Osama bin Laden for having 'caught the disease of screens, flashes, fans and applause.'

Mr. al-Suri's plans for a wave of "individual jihad" in the West are contained in "A Call to a Global Islamic Resistance," a 1,600-page book that he published on the Web in 2005, shortly before he was apprehended in Pakistan with a $5 million CIA bounty on his head. The manifesto combines strikingly clearheaded historical analysis with trenchant commentary on what he saw as two decades of strategic and operational failures by jihadists. The destruction of the World Trade Center was a short-term public-relations success for al Qaeda, Mr. al-Suri conceded, but American cruise missiles had made short work of the group's havens in Afghanistan, and Western special forces and intelligence agencies had decimated the ranks of its fighters and crippled the global jihadist movement.

What Mr. al-Suri learned from the Afghan debacle and from al Qaeda's subsequent defeat in Iraq was that jihadists were all but helpless in battle against modern Western armies. In place of old-fashioned hierarchical terror organizations, which had failed, he called for a global struggle in which shadowy motivators and facilitators would prompt jihadists to train and arm themselves in independent, self-generating terror cells that would target Western civilians. His goal: a relentless campaign of exemplary acts of violence under a single ideological banner, culminating in the use of weapons of mass destruction.

Mr. al-Suri has been credited by Western European intelligence agencies and police with drafting the blueprints for the train bombings in Madrid in 2004 and London in 2005, as well as with helping to shape Abu Musab al-Zarqawi's terror campaign in Iraq. Apprehended on Oct. 31, 2005 by the Pakistani intelligence service, he was turned over to the CIA, which sent him back to his native Syria. He was "treated well during his incarceration" of six years, according to an Islamist spokesman in London, Dr. Yasser al-Siri. Some Islamists place him now in Syria; others suggest that he was flown to Iran, where he resided along with other top figures in al Qaeda after the organization's defeat in Afghanistan, before he made his way to Pakistan.

One of the chief exponents of Mr. al-Suri's brand of jihadism was the Yemini-American cleric Anwar al-Awlaki, who helped to motivate Maj. Nidal Malik Hasan to kill 13 colleagues in November 2009 at the U.S. Army base in Fort Hood, Texas. Mr. Awlaki was also instrumental in producing a slick, downloadable English-language magazine called Inspire, which contained bomb-making recipes along with long excerpts from Mr. al-Suri's writings.

Mr. al-Suri is interesting, said Gilles Kepel, the French political scientist who is widely considered the world's leading authority on Islamist radicals, "because he is part of the second generation of the jihadist movement, the ones who were concerned with the failure of mobilization after 9/11." For Mr. al-Suri, according to Prof. Kepel, the failure of the 9/11 attacks to rouse global Muslim outrage was compounded by the failure of the jihadist terror campaign in Iraq, and by subsequent Western success in reducing what was once a global movement into increasingly isolated archipelagos of local movements and causes.

Mr. al-Suri held Osama bin Laden personally responsible for sticking to outmoded methods of organization and warfare that made al Qaeda easy prey for Western armies and intelligence services. His direct communications with bin Laden, some of which were recovered from a hard drive obtained in Kabul in 2001 by Journal reporter Alan Cullison, were characteristically blunt. "We are in a ship that you are burning on false and mistaken grounds," he wrote, accusing bin Laden of having "caught the disease of screens, flashes, fans and applause."

In a meeting of the leadership of al Qaeda held in northern Iran in 2002, as reported by Brynjar Lia of the Norwegian Defense Research Establishment, Mr. al-Suri issued a strong call for the movement to abandon its hierarchical structure or face annihilation by the West—a call that other senior jihadists ignored. He repeated his message in his magnum opus, warning that "If we insist on using these methods under the current circumstances, it is—in my opinion—like committing suicide." Now that al Qaeda has been crippled and its leadership killed or jailed, Mr. al-Suri appears to have won both the ideological and the practical battle.

Born in Aleppo, Syria, Mr. al-Suri is hardly a friend of the Assad family or the Syrian regime. He joined the Muslim Brotherhood while a student and then rose to become a member of the Brotherhood's military command in 1982 during the ill-fated uprising in the city of Hama, which was brutally crushed by Hafez al-Assad, the current president's father.

Mr. al-Suri fled to France and then to Madrid, where he met and married a Spanish woman named Elena Moreno Cruz, who converted to Islam and accompanied him on his subsequent journeys. Ms. Cruz furnished him with an EU passport, and he has remained, by several accounts, deeply in love with her.

In "Architect of Global Jihad," a scholarly biography of Mr. al-Suri published in 2008, the Norwegian expert Mr. Lia portrays him as the most brilliant and dangerous ideologue of his cohort of radicals, "a dissident, a critic and an intellectual" who put "hard-nosed realism before religious wish-fulfillment and pragmatic long-term strategies before utopianism"—qualities that would be admirable if he were not a master terrorist.

The fact that Mr. al-Suri is now free is likely to make his theories more visible and popular in jihadist circles, even if he doesn't succeed in planning or inspiring another mass casualty attack like the train bombings in Madrid and London. His release also serves as a reminder that Mr. Assad's continuing hold on power in Syria is a threat not only to his own people but also to the civilian populations of the West.