Belgian spies said attacks were coming, but overwhelmed security forces were powerless to prevent terrorists from killing 34 and wounding hundreds more.

FP: Fears of a possible terrorist attack in Brussels have been running high for months. The Belgian government went so far as to lock down the capital last year after receiving specific intelligence that a group of extremists were plotting a bombing and gun assault in the style of the Nov. 13 attacks in Paris that killed 130 people.

The lockdown was lifted after four days, but the government’s performance fueled criticism that it was behind the curve on the terrorist threat. Belgian security services also came under fire for having monitored and tracked several of the Paris attackers without informing their French counterparts about the suspects.

Concerns about Belgium’s ability to counter the terrorist threat were reinforced after it took authorities four months to track down the most wanted man in Europe, Salah Abdelslam, who was hiding in the Molenbeek neighborhood of Brussels. Abdelslam is believed to be the last surviving member of the Islamic State cell that carried out the devastating attacks on Paris, and authorities had repeatedly raided the neighborhood before apprehending him.

After Abdelslam’s arrest last week, Belgian officials said they had uncovered a much larger network than initially suspected. But as authorities scrambled to try to roll up the militants, an Islamic State bomb ripped through the departure area of Brussels’s main airport on Tuesday. An hour later, another bomb detonated at the busy metro station near the European Union headquarters. The attacks killed 34 people and wounded nearly 200. The Islamic State claimed responsibility, singling out the “crusader” nation of Belgium and promised that more attacks were coming to countries allied against it.

As police in Brussels launched a manhunt for one of the suspected assailants, Belgium and its allies, including the United States, faced a number of crucial, unanswered questions in the wake of the attacks: Who is the suspected bomb-maker, or bomb-makers, behind the killings, and where are they located? How sophisticated were the ingredients used in the explosives? How did the extremists evade detection? And just how big is the terrorist network that helped fund, plan, and carry out the massacres?

Counterterrorism experts said Belgium now finds itself confronting a terrorist infrastructure that has planted deep roots in Belgian society as the war in Syria has escalated. According to figures assembled by Belgian terrorism researcher Pieter Van Ostaeyen, as many as 562 Belgians have fought in Iraq or Syria. At least 124 hail from Brussels. With a population of 11 million, Belgium has sent more fighters per capita than any country in Europe to the charnel house of the Syrian civil war. According to Van Ostaeyen’s figures, at least 75 Belgians have returned to the country — making it nearly impossible for the country’s small and under-budgeted security services to track them all.

“The bigger problem is that such a thriving terrorist infrastructure was allowed to establish itself in the first place,” said Bruce Hoffman, the director of the Center for Security Studies at Georgetown University and a counterterrorism scholar. He added that “key operatives and senior [Islamic State] commanders are based in Brussels,” where they’re able to draw “on a network of operatives and also sympathizers.”

Last year, a Belgian court convicted 45 members of the radical Salafi group Sharia4Belgium on terrorism charges. Judge Luc Potargent, presiding over one of the largest terrorist trials in Belgium’s history, found that the group’s leader, Fouad Belkacem, had “prepared young people physically and psychologically for armed combat” and sentenced him to 12 years in jail. According to Van Ostaeyen’s figures, 79 individuals with links to Sharia4Belgium have taken up arms in Syria and Iraq. The group has been identified as a main supplier of Belgian fighters to those two countries.

Another prominent recruiter, Khalid Zerkani, sent at least 45 individuals to fight in Iraq and Syria, according to Van Ostaeyen. Among his recruits was Abdelhamid Abaaoud, whom authorities describe as the ringleader of the November attacks in Paris. Last July, a Belgian court sentenced Zerkani to 12 years in jail for his role in sending fighters to Syria.

The alarming reach of the Islamic State has prompted soul-searching in Brussels and accusations that the government is flailing in its counterterrorism efforts. Belgian Prime Minister Charles Michel last year pushed back at the criticism and announced a series of new security measures, including introducing ankle bracelets to monitor extremists and extending detention periods for terrorism suspects not yet charged in a crime.

In February, Belgian officials unveiled plans to double spending to about $446 million on law enforcement and intelligence. The plan also called for adding a thousand police officers by 2019 to a city district that includes Molenbeek, a working-class neighborhood that has become notorious for its role as a hub for Islamist militants.

Molenbeek poses a significant challenge to law enforcement officials, according to Matthew Levitt, a counterterrorism expert who recently traveled to Belgium to study its extremism problem. He said local authorities in Molenbeek lack the resources to get ahead of the “radicalization curve” in the densely packed district that suffers from high unemployment.

“It’s great that they have a counter-radicalization cell within the police, but it’s eight people,” he told reporters in a teleconference.

The Molenbeek police added 50 new officers following the Paris attacks, but Levitt noted that its total pool of officers had been down by 185 officers due to recruiting problems. “It’s hard to recruit people to go to work in Molenbeek,” he said. “It’s a tougher neighborhood.”

Levitt said authorities are spread thin given that Molenbeek alone has approximately 47 mosques and the vast majority of imams in Belgium do not speak local languages, making them less than ideal partners for counter-radicalization programs. Philippe Hayez, a former official with the DGSE, the French counterpart to the CIA, described the Belgian security services as a small but professional community, comprising fewer than 1,500 people.

With extremist networks having become entrenched over a period of years, Belgium and its neighbors are confronted by a threat beyond what security services had ever anticipated. “I think they have a deeper and more pervasive network than we’ve imagined in Europe, and it’s a network that operates according to its own timetable and seeks to take advantage of vulnerabilities and opportunities as they present themselves,” Hoffman said, reflecting on what the Brussels attacks have revealed about the Islamic State’s capabilities. Hayez called the problem “Euroterrorism nouvelle vague” that “isn’t only a threat from abroad like it was with al Qaeda but a mix of external and internal threats.”

Current and former senior U.S. national security officials said Belgium had been slow to wake up to the threat — until the Paris attacks, which were planned, in large part, from a Brussels cell. But they said it remained unclear whether recent intelligence warnings contained details of potential attacks or more general language that was too vague for police to act on.

After the November Paris attacks, a Belgian parliamentary committee launched an inquiry as to why security services were unable to “connect the dots” of information on the attackers.

Despite belated efforts to bolster resources to take on the Islamic State, Belgian authorities are facing a highly capable foe. Islamic State operatives in Europe have repeatedly shown a high proficiency in communications security, including the use of one-time, or “burner,” phones and the use of encrypted communication tools. They have relied on high-quality forgers to provide fake documents and used Europe’s open borders to freely travel the continent plotting mayhem. They have amassed stockpiles of weapons and explosives and have deployed their arsenal in meticulously planned suicide attacks.

“We were told by intelligence agencies that these types of attacks were a thing of the past, that enough red flags would go up when these guys were constructing their suicide vests that they would be noticed,” said Robin Simcox, a terrorism and intelligence expert at the Heritage Foundation. Rather, lone-wolf attacks by disaffected militants were supposed to be the terrorism phenomenon of the future. Mass casualty attacks requiring “broader networks” were so “operationally compromised” that they would be picked up by the police, Simcox said.

Or so the story went, anyway, Simcox said. “That was blown out of the water by Paris in November, and now it’s been hammered home today.”