MADRID — Rachid Wahbi came to Syria from a Spanish slum, rushing toward death.

And he didn’t plan to die alone.

Facing a camera hours before the end, the bearded, 33-year-old cabdriver wore a black headdress and a black flak vest and held an AK-47 rifle. He spoke in hesitant classical Arabic with a north Moroccan accent. He said he had studied his target and, God willing, his action would end in triumph. He wished the glory of martyrdom for his fellow fighters in the al-Nusrah Front, al-Qaida’s Syrian branch.

When the cameraman asked about his mother, the Spaniard became emotional.

“I want to thank my mother because she inspired me,” Wahbi said, according to a translation by the Spanish national police. “Mother, you must be happy because God will reward you.”

The al-Nusrah propaganda video shows Wahbi disguised in the helmet and uniform of a Syrian soldier as he hugs a comrade and climbs into a truck packed with explosives. The truck bears down on an army outpost. An explosion thunders. A column of smoke, seen from multiple camera angles, climbs toward the sky.

Wahbi killed 130 people in that suicide bombing on the al-Nairab military base in northern Syria on June 1 of last year, according to Spanish authorities. And the numbers get grimmer.

Five holy warriors from Spain have died in Syria, three in bombings that killed another 100 people, police say. Last month, Spanish police stormed the hillside ghetto where Wahbi lived in Ceuta, a Spanish territory in North Africa, and arrested a ring of extremists who are charged with sending as many as 50 fighters to Syria. Indicating a threat much closer to home, the accused leader had previously been acquitted of plotting attacks on targets in Spain with a group linked to al-Qaida and a former Guantanamo inmate.

“The global jihad has prioritized the Syrian conflict as its principal front,” said a top Spanish intelligence official who requested anonymity because of the continuing investigation. “And it has directed its subsidiaries to move combatants to the zone. What worries us is that this experience could serve as preparation, as training to return to European countries and carry out attacks at home.”

Hundreds of Europeans and thousands of other Sunni Muslim foreign fighters have made Syria the new land of jihad. The migration complicates an already delicate calculus in Washington and in European capitals that are aiding the fractious rebel coalition in Syria. European security chiefs see the flow of extremists to and from Syria as their top terrorist threat. It also raises concerns that European militants radicalized by or returning from the Syrian conflict could strike U.S. targets overseas or travel across the Atlantic.

“Imagine this: Between 2001 and 2010, we identified 50 jihadists who went from France to Afghanistan,” said a senior French counterterror official who also requested anonymity. “Surely there were more, but we identified 50. With Syria, in one year, we have already identified 135. It has been very fast and strong.”

The statistics are even stronger in adjoining Belgium, one-sixth the size of France. Between 100 and 300 jihadis have journeyed from Belgium’s extremist enclaves to Syria, according to a veteran Belgian counterterror official. Other significant fighting contingents represent Britain, Denmark, the Netherlands, Canada, Central Asia, Libya, Tunisia and Saudi Arabia. The senior French official estimated the total number of Europeans to be at least 400. Others say it could be double that, but counterterror officials warn that precise numbers are difficult to establish.

The foreign fighter phenomenon in Syria “is one of the things that most worries a number of European government agencies,” Italian Defense Minister Mario Mauro said in an interview. “But this is also within the reactive capacity of a system built by democracies, therefore based not on preventive arrests but on monitoring and intelligence activity to prevent situations like this from degenerating.”

The total number of the rebel forces – Syrians and foreigners, full-time and occasional fighters – is thought to be in the tens of thousands. Estimates range from above 60,000 to below 100,000, based on interviews with U.S. and European officials and experts.

A recent private report examines the role of Sunni foreign fighters who have converged from across the Muslim world to battle the regime of Bashir Assad and his powerful Shiite allies, Hezbollah and Iran. Foreign fighters account for up to 10 percent of the rebels in the data sample examined by the study, which relies on sources including online obituaries of militants and social networks and is titled “Convoy of Martyrs in the Levant.”

Released last month by Flashpoint Partners, a New York security contracting firm, and the Washington Institute for Near East Policy, a think tank that tends to align with Israeli views, the report compares the conflict to previous arenas that attracted extremists: Afghanistan, Pakistan, Iraq, Yemen.

“At the very least, the current war in Syria can be considered the third-largest foreign mujahideen mobilization since the early 1980s — falling short only of Afghanistan in the 1980s and Iraq during the last decade,” the study concludes. “[T]he mobilization has been stunningly rapid — what took six years to build in Iraq at the height of the U.S. occupation may have accumulated inside Syria in less than half that time.”

Syria is familiar turf that once served as a hub for militants en route to fight in Iraq. It is closer and more accessible to Europe than other jihadi destinations: Militants travel by air or land to Turkey, where smugglers sneak them across the border. There is little interference by authorities in Turkey, a major sponsor of the Syrian rebels.

Despite the ferocity of the civil war, Syrian cities offer better living conditions to foreign volunteers than al-Qaida’s remote compounds in Pakistan or the impoverished wastes where Islamists operate in Somalia and Mali. The ever-improving technology of the Internet and mobile phones allow combatants to trumpet their exploits and remain in close communication with comrades back home.

“There are guys who regularly update their Facebook pages from Syria,” said Claudio Galzerano, the chief of the international terrorism unit of the Italian police in Rome.

Moreover, the cause enjoys unique popularity. Many Sunnis and non-Muslims alike regard it as a crusade to overthrow a brutal dictator who uses chemical weapons to slaughter his people. The Obama administration and European governments support the Syrian opposition and the Supreme Military Command, which encompasses the Free Syrian Army and other relatively moderate groups. The Convoy of Martyrs study describes “Arab Spring-motivated, pro-democratic revolutionary fervor” that pushes foreign volunteers to join the Free Syrian Army, rather than extremist rebel units.

Nonetheless, secular idealists are a minority among the foreign fighters, according to European counterterror officials. The octopus-like embrace of anti-Western, al Qaida-connected networks in Europe and the Muslim world— sometimes led by the same chiefs as in past conflicts — has shifted to Syria. Many foreign recruits join al-Nusrah or the Islamic State of Iraq, al Qaida allies that field some of the toughest fighters. These Sunni Islamist groups clash with other rebel factions and argue among themselves about whether to widen their jihad beyond the borders of Syria, according to counterterror officials.

European police fear that well-trained, battle-hardened veterans will return from Syria and, on their own or acting on orders from terrorist bosses, decide to continue the war. Western leaders say they are taking pains to prevent stepped-up aid to the Syrian opposition from reinforcing the extremists. When the European Union ended an arms embargo to the rebels in May, reluctance about that decision resulted partly from concerns that the weapons would end up in the wrong hands.

“There is a risk, and how,” said Stefano Dambruoso, an Italian parliamentary deputy who is a former top anti-terror prosecutor. “In a situation that is out of control like the one in Syria, it is really very dangerous. Italy supported maintaining the embargo because really we don’t know who we are dealing with. The rebels are still not clearly identifiable.”

Dambruoso knows the treacherous turf. Based in Milan in the early 2000s, he led prosecutions of al Qaida operatives involved in plots in Europe and linked to the assassination of Ahmed Shah Massoud, a legendary anti-Taliban commander in Afghanistan, just two days before the 9/11 attacks on New York.

Several cases in Italy involved the Tunisian Combatant Group, part of al Qaida’s terrorist coalition. Some Italian cells sent recruits via Syria to join the insurgency in Iraq. Key operatives were arrested by Italian police and then deported to prisons in Tunisia, sometimes after serving time in Italy.

After the Tunisian revolution ended in 2011, however, a new government in Tunis released convicted terrorists and allowed them to create a radical Islamic party, Ansar al Sharia. It is led by Seifallah Ben Hassine, who is a founder of the Tunisian Combatant Group and a former ally of Osama bin Laden, according to European and UN counterterror officials and documents. His party has been recruiting and deploying holy warriors to the Syrian front from camps in the south of Tunisia, according to European investigators.

Tunisians account for 16 percent of foreign fighters in Syria, the second-largest group there, according to the data sample in the Convoy of Martyrs Study.

“This is one of our top concerns,” said Galzerano, the Italian police commander. “They are sending a lot of Tunisians to Syria. Everyone is welcomed by the rebels, including those who have little skills or experience.”

Syria holds another attraction for aspiring holy warriors: It serves as a refuge from law enforcement. Some Europeans in Syria are seen as active threats to their homelands. An illustrative case began last September when someone threw a hand grenade at a kosher grocery store in a suburb north of Paris, wounding one person. Traces of DNA on the grenade led French investigators to a known Islamic radical and revealed a dangerous network operating in three cities.

When a police tactical team raided the Strasbourg apartment of the suspected leader, a Muslim convert of French-Caribbean descent, he opened fire and died in the ensuing shootout, authorities said. Police made a dozen arrests and discovered a garaged stockpile of explosives, including pressure-cooker bombs like those used in the Boston Marathon attack.

“We learned they were planning a campaign of attacks, including car bombs,” said the senior French counterterror official. “They wanted to launch the attacks, then flee to Syria and fight there. Three of them were able to escape to Syria.”

The three suspects who fled the French manhunt joined the al-Nusrah front. One has been badly wounded in combat against the Syrian military, according to the French counterterror official.

Western investigators track the communications and travels of foreign fighters because of their proven capacity for violence and potential contact with al Qaida and its affiliates, who hate the West as much as they hate Assad, investigators say.

“We hear threatening rhetoric in the intercepts,” a European police commander said.

The presence of a minority of hardcore Islamic terrorists in the insurgency poses a conundrum when it comes to Western intervention. One school of thought urges restraint in order to avoid creating a monster comparable to the U.S.-backed Islamists who fought the Soviets in Afghanistan in the 1980s and then morphed into al Qaida. Others, in contrast, believe the West could influence the Syrian rebel movement by doing more.

A former CIA counterterror chief leans in the latter direction. Author Charles (Sam) Faddis served in South Asia and the Middle East, where he led clandestine CIA operations in Iraq that preceded the U.S. invasion in 2003. He communicates periodically with leaders of the Free Syrian Army and thinks the Western support is “too little, too late.”

“I’m the first guy who parts company with the neo-cons (neo-conservative Republicans in Washington) who think we should get involved everywhere,” Faddis said. “I’m against putting American troops in there, and I’m against a no-fly zone. But our approach has been short-sighted.”

There is a real threat of a blowback against the West even from a relatively small number of trained, combat-hardened veterans of the conflict, Faddis said. But he criticizes the Obama administration for not having moved quickly to provide arms and intelligence to the Free Syrian Army.

“You were going to have extremists flocking in there anyway,” he said. “Now you’ve increased their influence. Their power has been enhanced by our not getting involved in a more significant way. We need to get on the ground, map the terrain, figure out who we can work with.”

Last month, President Obama authorized providing small arms and training to Syrian rebels to augment nonlethal aid that they already receive. The U.S. government is working hard to support the pro-democracy forces and thwart al-Nusrah, which the U.S. designated as a terrorist group last year, and other extremist groups, State Department officials say.

“We remain deeply concerned by the violent extremism there,” said a State Department official who requested anonymity because he is not authorized to speak publicly. “We distinguish between those in the opposition seeking a moderate, democratic Syria and those who are trying to hijack it. We make clear with the armed opposition leaders who don’t espouse these [extremist] ideals the importance of isolating the extremists, so it doesn’t take root in the future Syria they are trying to fight for.”

The U.S. has committed $250 million in nonlethal assistance to the rebels and $815 million in humanitarian aid to those affected by the conflict, according to a State Department fact sheet. In areas of Syria under rebel control, the U.S. attempts to shore up the democratic opposition by helping local governments deliver security and other essential services, providing material such as trucks, communications equipment and computers. U.S. officials put the recipients through a vetting process intended to prevent aid from going to the extremists, the State Department official said.

“It’s important that the vetting is in place precisely because there are groups like al-Nusrah trying to intercept things,” the official said. “Sometimes there’s a delay as a result.”

In Europe, authorities have a hard time identifying and prosecuting suspected jihadis for terrorist activity when they return from Syria. Some known extremists insist they fought in the Free Syrian Army, which they indignantly point out has the backing of President Obama, French President François Hollande and others. Judges are more skeptical of the prosecutions than they were with defendants returning from Afghanistan or Iraq, counterterror officials say.

Courts in Europe often struggle to find enough evidence to lock up Islamic extremists if their alleged crimes center on ideological activity or combat in foreign countries.

Raphael Gendron is an example. In late 2008, Italian police arrested Gendron, a Frenchman residing in Belgium, and Bassam Ayachi, a Syrian-Belgian imam, in a camping vehicle coming off a ferry from Greece in Bari, a city at the heel of the Italian boot. Police discovered five illegal immigrants and a trove of jihadi propaganda in the vehicle.

In 2006, Gendron had been convicted of a charge of inciting hate and violence against Jews with Internet propaganda in Belgium. Ayachi had performed the marriage in Brussels of the Tunisian suicide bomber who later killed Massoud in Afghanistan, investigators say. Both had longtime ties to networks that had been implicated in terror plots and had sent jihadis to Bosnia, Afghanistan, Pakistan and Iraq, according to investigators and Italian court documents.

An Italian court convicted the duo of acting as recruiters and operatives for al Qaida, but an appellate panel freed them last year. Soon Gendron went to Syria to join a rebel battalion commanded by Ayachi’s son, a veteran of the Belgian military, according to Belgian investigators. In April, the 37-year-old Gendron died in combat near Homs, Syria.

Most suspects in past al Qaida-related terrorist plots against the West traveled first to jihadi combat theaters, and many were European or spent time in Europe. The combat zones and training venues of Pakistan and Afghanistan generated a stream of militants intent on striking the West — from the Sept. 11 hijackers to the failed Times Square bomber in 2010.

Fears of massive blowback against Western nations from Iraq did not materialize, however. The Iraqi conflict certainly played a role in radicalization. But some European jihadis who returned from Iraq told investigators that, despite their eagerness to fight in a war zone, they would not commit violence against civilians at home.

The background of foreign volunteers determines the reception they get from Syrian extremist groups, investigators say.

“We see a little of everything in the profile of the recruits,” the top Spanish intelligence official said. “There are people who are clearly with al Qaida, or are associates of its subsidiaries. Then there are people who have no connection with anything. Solitary actors inspired to go to there and fight.”

Militants with useful skills, such as medical professionals or computer experts, are kept out of combat and given support roles. Men with military experience deploy in front-line units.

Those with little to offer quickly become human bombs.

Wahbi, the Spanish suicide attacker, died soon after his arrival in Syria. He had no criminal record. Also known as Rachid Mohamed, he had supported his wife and children driving a white Mercedes taxi in Ceuta, one of two Spanish cities on the Moroccan coast. His predominantly Muslim neighborhood, known as El Principe, resembles a Brazilian favela or a North African casbah: The slum sprawls over a canyon near the Moroccan border and serves as a fortress for organized crime and Islamic extremism.

In 2006, 300 Spanish police officers raided El Principe, a show of force planned for the rough topography and hostility to law enforcement. Police rounded up 11 suspects accused of belonging to an al Qaida-linked group that allegedly plotted to attack a military base and a fairground in Ceuta and discussed joining the jihad in Iraq or Afghanistan. The suspects included an accused ideologue known as “Marquitos” and two brothers of a Spaniard once imprisoned in the U.S. base at Guantanamo Bay, Cuba.

However, the prosecution ran into problems with turning intelligence into evidence and presenting the testimony of two protected witnesses. The trial ended last year in acquittals. There were cries in the media and Muslim community that innocent men had been railroaded.

Spanish prosecutors now have filed an indictment alleging that the accused ideologue never relinquished his command role in Ceuta’s Islamic underworld. After war broke out in Syria in 2011, Marquitos allegedly recruited men from Ceuta and neighboring Morocco to join the new jihad, according to Spanish intelligence officials.

The taxi driver and two friends were among the first recruits. They departed in April of last year, flying via Malaga and Madrid to Istanbul, where smugglers helped them enter Syria and join al-Nusrah.

“They were in Syria very few days,” Wahbi’s widow, Sanaa, told El País newspaper last year. “Maybe not even a week. During the trip, which lasted a month and a half, he communicated with us by Messenger (internet chat). They were in Turkey quite a while because it seems they couldn’t reach Damascus. When they arrived in Syria he called us, but he didn’t give us details of what he was doing.”

Wahbi’s attack stands out as one of the war’s deadliest. Police say the ring in Ceuta sent at least 20 and up to 50 recruits along the same route or via Morocco. The sophisticated operation paid for travel and provided funds to widows and children of fallen fighters. Police are still trying to determine if the financing came from the criminal activity such as the drug trade, according to Spanish intelligence officials.

On June 21, authorities launched another raid on El Principe. Four hundred officers of the police and Guardia Civil participated, backed by a helicopter hovering over the densely populated canyon. Police once again arrested Marquitos, now 39, and seven accused accomplices. They are awaiting trial.

Police believe the clandestine flow to Syria continues from European hotbeds of extremism.

“There are two categories,” said a Spanish intelligence official who requested anonymity because of the continuing investigation. “Those who go intending to die quickly in a suicide attack. And there are those who want to participate in an act of jihad, taking a great risk because they are going to acquire contacts, training and experience. They want to fight, survive and return. Those are the ones who worry us the most.”