PARIS — Before a SWAT team stormed a tenement in the Belgian city of Verviers in January, police used listening devices to monitor their targets inside: Belgian jihadis who had returned from Syria to attack a local police station in the name of the Islamic State.

Police gunned down two suspects during the pre-dawn firefight, foiling the plot. But a chilling detail stuck with the Belgian counterterror investigators who tracked down the plotters with help from French and U.S. intelligence. As investigators listened, the militants responded to the police assault with a ferocity forged in the battlegrounds of the Middle East.

“They were talking about their plans to commit violence here,” a senior Belgian counterterror official recalled in a recent interview. “The police flashbang grenade goes off. And immediately these two start firing their AK-47s. No hesitation, no panic. These are guys with combat experience. They were ready to fight and die.”

As the fast-paced investigation of the rampage in Paris that left at least 129 people dead unfolded, elite tactical teams carried out another pre-dawn raid Wednesday on suspected terrorists holed up in an apartment outside the French capital. The target was the accused Belgian mastermind of the thwarted effort to attack the police station in Belgium in January who is also believed to have played a central role in directing the Paris attacks last week: Abdelhamid Abaaoud.

Two suspects died in the gunfight this morning, one of them a woman who detonated a bomb vest, authorities said. Five SWAT officers were wounded. Police arrested five suspects. The target of the raid was Abaaoud, who investigators now believe may have made a daring return from the Islamic State’s stronghold in Syria to lead the Paris attacks in person. Authorities had not yet announced Wednesday morning whether he was among those killed or captured, or if he remained at large. (Update: Police later confirmed that Abaaoud died in the raid.)

Abaaoud, 27, was a stick-up man-turned-terror kingpin from the tough Brussels suburb of Molenbeek, which has been raided repeatedly by Belgian counterterrorism investigators in the days since the attack. The extent of his role in the Paris massacre is not yet clear, but he had longtime links to at least two of the suspected attackers, according to European counter-terror officials.

Abaaoud’s name had already surfaced in connection with previous plots targeting France and Belgium. In one instance that directly foreshadows Friday’s attack in Paris, French police in August arrested a militant who had trained in Syria. He told authorities that Abaaoud had directed him to attack live music venues in France, officials say.

There are also suspicions that the Belgian was involved in a deadly shooting at the Jewish museum in Brussels last year, as well as the foiled attack on a Paris-bound train from Belgium by a Moroccan gunman who was subdued by a trio of vacationing Americans this summer.

The leading role of Belgians in the Paris massacre highlights the disproportionately large shadow cast by Belgium on the map of terror in Europe during the past two decades. Belgium featured in a wave of bombings in France by Algerian-dominated groups in the 1990s. Belgium-based terrorists have been active in al Qaida: killing an anti-Taliban warlord in Afghanistan two days before the Sept. 11 attacks, plotting to bomb the U.S. embassy in Paris, and sending jihadis to Pakistan, Africa and U.S.-occupied Iraq in the 2000s.

In a practice seen again in the Paris plot, operatives in the Franco-Belgian networks move back and forth across the border with speed and agility, outpacing law enforcement.

“Things are easier for terrorists in Belgium than they are in France,” said Commandant Mohamed Douhane of the French national police. “They use Belgium as an outpost.”

Mounting Threats, Multiple Faces

Friday’s tragedy in Paris was an attack foretold. During interviews earlier this year, French and Belgian terror chiefs warned that a swarm of threats had reached overwhelming levels. They identified Abaaoud as one of several senior Francophone militants relentlessly plotting attacks on Europe from Syria.

“The threat is so high,” a French counterterror chief said during an interview in the spring. “There will be new attacks. There is a permanent fatwa from the Islamic State: Attack the West.”

As disturbing intelligence reports piled up in recent months, French and U.S. counterterror agencies teamed up to target suspected European plotters. Complicating matters, the threat had multiple faces. Al Qaida in Yemen had overseen the attack on the offices of Charlie Hebdo magazine in January. Although the Islamic State has many more recruits than al Qaida’s affiliate in Syria, the latter group includes veterans who have been hatching plots against Western targets since the early 2000s, when they operated from refuges in Afghanistan and Pakistan.

“They are a direct threat and, while smaller than the Islamic State, have bigger plans,” the French counterterror chief said. “They want to do more spectacular attacks, [a] more choreographed style of attacks as opposed to shootings.”

U.S. drone strikes this summer killed two top names on the al Qaida list who kept French spymasters awake at night: convert David Drugeon, an expert bomb-maker, and Said Arif, who had been linked to plots against France dating to 2000.

“There has been some progress made in getting guys with strong connections and who were among the most operationally capable,” a U.S. counterterror official said. “But clearly the bench is pretty deep.”

Air strikes also targeted Abaaoud and two Frenchmen thought to be actively involved in Islamic State plotting against France, according to U.S. and European counterterror officials. In October, a French bombing raid on the Syrian city of Raqqah missed Salim Benghalem, a 31-year-old Parisian ex-convict known for beheadings and sadistic treatment of hostages. Another Islamic State Frenchman who dodged an air strike was Boubaker el-Hakim, who is suspected of assassinating two political leaders in Tunisia in 2013. Both jihadis have ties to the Charlie Hebdo attackers.

About 2,000 French militants have gone to Syria, the single largest contingent of fighters from Europe. French-speaking Tunisians and Moroccan militants in Syria are thought to number close to 10,000. But the more than 500 Belgians are the largest proportionate group of Europeans. Most Francophone jihadis join the ranks of the Islamic State in Syria, where they live and fight together. They see France as their top target.

For ISIS, Shifting Strategies

The Islamic State’s war on the West differs from the hands-on plotters of al Qaida, whose foreign operations unit has traditionally hatched plots in Pakistani and Yemeni hideouts and directed attackers to their targets. Those plots often involved bombs and specific, highly symbolic targets. Instead, the primary focus of the Islamic State, whose leaders are mostly Iraqi and Syrian, has been conquest of turf and the consolidation of their self-declared caliphate.

The Islamic State has used a social media barrage to inspire jihadis abroad to carry out strikes without training or direct contact. The group has also given its trusted foreign fighters considerable autonomy to develop attacks in the West, delegating details such as target selection to militants who best know their homelands, according to European and U.S. intelligence officials.

“The Islamic State’s general directive has been to do attacks,” the French counterterror chief said, “and the Europeans propose projects.”

This year, however, that dynamic seems to have evolved in response to an offensive by the coalition fighting against the Islamic State, according to U.S. and European counterterror officials. They said the Islamic State has developed a kind of external operations unit that may be behind a flurry of large-scale attacks in Paris, Egypt and Turkey, officials said.

“Months ago they created a department to coordinate the jihad overseas based on the foreign fighter elements,” a senior Spanish intelligence official said. “They weren’t as interested in that before. They were interested in the territory.’’

‘They Are Ruined People’

Belgium — small, prosperous, tolerant — has historically been a hub for espionage, arms trafficking, organized crime and extremist activity. The country has a generous welfare state and lacks the huge public housing projects that breed crime, alienation and extremism in France. Nonetheless, the integration of Muslims in Belgium remains problematic. Successive jihads in Afghanistan, Iraq and Syria have radicalized scores of young, disaffected, working-class Muslims. Most are of North African descent and have criminal pasts; the groups they join grew out of longtime networks active in Europe and the Muslim world.

Belgium has skilled counterterror officers who know the extremist underworld, including a number of investigators of Muslim descent. Despite the intensity of the terror threat, the bureaucracy puts constraints on them. The government has scrambled to beef up counterterror forces in recent years, with one unit tripling in size. It is hard to keep suspects in jail without overwhelming evidence, and sentences for terrorism are short — as in the rest of Europe.

In an interview, a senior Belgian law enforcement official said the swagger and savagery of the Islamic State has a disturbing appeal among aimless young criminals in Molenbeek and other neighborhoods.

“They go to Iraq and Syria because there they will be somebody,” he said. “Here they are nobody. They are told that if they join the Islamic State they will get to drive a nice car, get women, they won’t have to pay in the shops down there. They will be badass warriors.”

The Belgian official described a police search of the home of three brothers who all joined the Islamic State and have been implicated in decapitations and other violence in Syria. Their father had a well-paid job with a U.S. automotive company. Each brother had his own room stocked with computers, video games, clothes and other consumer goods, the law enforcement official said.

“They don’t work; they live with their family into their 20s,” he said. “They manipulate the welfare system for money; they don’t study. They go to Syria, and they come back with PTSD. They come back after they saw killing and raping. What are you supposed to do to cure them? They are ruined people. Game over.”

Rise of a Paris Plotter

Abaaoud’s trajectory is emblematic. He is of Moroccan descent, a wiry man with an engaging grin. Like many youths in Molenbeek, he got involved in low-level gangsterism and was arrested for a hold-up along with Salah Abdeslam of Molenbeek, who is now a fugitive suspected of renting cars and safe houses for the three Paris attack teams. Abaaoud also had ties to Abdeslam’s brother, who would die in one of the Paris suicide bombings.

Abaaoud joined the Islamic State and went to Syria, where he became notorious for a video in which he hauled a pile of corpses with a tractor and joked about it. In late 2014, intelligence agencies picked up communications indicating he wanted to carry out an attack back in Belgium. U.S., Belgian, French and German intelligence tracked the plotters for three or four months, officials say.

“The Belgians proposed an action to Daesh [Islamic State], and they said yes,” the senior French counterterror official said. Islamic State bosses provided $5,000 to help finance the operation, Belgian investigators said.

Abaaoud dispatched Sofiane Amghar, 26, and Khalid Ben Larbi, 23, who had fought in a special squad of fighters in Syria, according to Belgian investigators. Amghar, a Molenbeek recruit, posted a fake obituary about himself online to cover his tracks as he made his way back. Ben Larbi returned via the United Kingdom. They set themselves up in a safe house in Verviers.

Their plot involved using stolen police uniforms to storm a police station in the Brussels area. Three plotters stockpiled weapons in the safe house, monitored by police. The SWAT team went into action because an attack seemed imminent, officials said.

“We heard them speaking about projects and manipulating weapons, it was obvious they were about to do something,” a Belgian law enforcement official said. “One of them always stayed awake, standing guard. The stun grenades went off at the front room window, but they were lucky because they were in back and weren’t stunned. The firefight lasted 10 minutes.”

Abaaoud, however, had been directing his fighters by phone from Greece. He melted away. And if the allegations are true, he kept launching human missiles at France until his dreams of devastation came true on a Friday night in Paris.