French authorities say they believe Abdelhamid Abaaoud, a 27-year-old Belgian man, masterminded the November 13 attacks in Paris.

The focus on Abaaoud helps emphasize how tiny Belgium has taken on an oversized role in the European theater of jihad. The country has provided a steady flow of fighters to ISIS in the Middle East—including Abaaoud—and has been the site of planning of attacks in Europe. (The Daily Beast has a good timeline of incidents involving Belgian militants.)

Abaaoud was already suspected of planning a prior attack that was foiled by Belgian authorities in the days after January’s Charlie Hebdo attacks in Paris. Two suspects were killed in the operation. At the time, Slate’s Joshua Keating warned: “The Belgian police may claim today to have ‘averted a Belgian Charlie Hebdo,’ but it’s clear that the country’s radicalization problem is much larger, and will take more than police raids to address.” Those words proved prophetic.

Belgium has just 11 million people, and Pew estimated that about 6 percent of the population was Muslim as of 2010. But Belgian and French nationals make up around a quarter of the Europeans who went to fight in Iraq in the mid-2000s. While the government has acknowledged that hundreds of Belgians have gone to fight with ISIS or for other groups in the Syrian civil war, Pieter Van Ostaeyen, an independent researcher, calculated in October that 516 Belgians had fought in Iraq or Syria, far higher than the government’s figures. Based on his numbers, Belgium has contributed more fighters per capita to the fight in the Levant than any other European country.

The central figure in Belgian militant Islamism is Fouad Belkacem, a 33-year-old preacher and founder of the group Sharia4Belgium. He was born in Belgium to Moroccan parents, and is a disciple of the British radical Islamist Anjem Choudary. Belkacem, who had been arrested for various petty crimes, organized burnings of American flags after 9/11 and harassed gay Muslims. Sharia4Belgium became a major feeder for fighters in Syria and Iraq, including Jejoen Bontinck, whom Ben Taub profiled in an excellent New Yorker story. Bontinck, a former TV dance-show celebrity, was a convert, as many of the most prominent European ISIS fighters have been. (Van Ostaeyen calculates that only 6 percent of Belgian-national ISIS fighters are converts, however.) Others, like Abaaoud, came from secular or mildly observant Muslim families, but became radicalized. In December 2014, Belkacem and 45 other members of Sharia4Belgium were found guilty of membership in a terrorist group. He is serving a 12-year prison sentence.

Belgian jihadism seems to mimic French Islamist militancy, only more concentrated—as befits the smaller country. Both have large numbers of immigrants who are poorer and isolated from the dominant culture. Both countries have also seen far-right, anti-immigrant parties rise by loudly declaring a Muslim menace. Experts also say it is comparatively easy to acquire illegal guns in Belgium, making it an attractive base for operations. The Washington Post notes that Belgium’s unusual bilingualism—Flemish and French—makes it hard for immigrants who only speak French to find work and assimilate. And deep distrust between French- and Flemish-speaking government officials has created an elaborate and sclerotic security apparatus that doesn’t always deal with threats efficiently and promptly.

In particular, Belgian jihadism is concentrated in Molenbeek. It’s a neighborhood of nearly 100,000 people in Brussels, northwest of the city center, which has had a large Muslim population for many years. There are 22 known mosques in the district. Molenbeek shares some characteristics with the banlieues in French—densely populated, large immigrant populations, very high unemployment, complaints of inadequate government services, isolation from the central city and corridors of power. In other ways, however, Molenbeek is rather different: It has a strong middle class, bustling commercial districts, and a gentrifying artist class.

“I notice that each time there is a link with Molenbeek,” Prime Minister Charles Michel said Sunday. “This is a gigantic problem. Apart from prevention, we should also focus more on repression.” (That unfortunate wording may be a failure of translation.) Interior Minister Jan Jambon added: “We don’t have control of the situation in Molenbeek at present.”

Jambon’s statement has reawakened concerns about Muslim “no-go zones” in European cities. It’s an idea Bobby Jindal, the Louisiana governor and Republican presidential hopeful, was heavily pushing in January. According to the myth, there were large zones that police forces had simply ceded to sharia gangs, and into which neither non-Muslims nor law enforcement dared to go. When I looked into it at the time, there was no evidence that such areas actually existed. Even Daniel Pipes, a leading alarmist about the threat of radical Islamism, argued that Jindal was mistaken in describing the banlieues that way. But there were disturbing reports of gangs of men intimidating or beating non-Muslims or residents whom they deemed insufficiently observant.

Does Molenbeek prove that no-go zones are real? On the one hand, there’s Jambon’s statement. But in other clear ways, Molenbeek doesn’t appear like a Muslim exclave in the middle of Brussels. Jambon himself pointed this out in his remarks on TV on Sunday, mentioning Molenbeek’s non-Muslim mayor and its own constabulary: “I see that Mayor Françoise Schepmans is also asking our help, and that the local police chief is willing to cooperate. We should join forces and ‘clean up’ the last bit that needs to be done, that’s really necessary.” Journalists seem to be having little trouble reporting from the area; Politico sent a reporter and photographer out in the neighborhood, and found that while residents were not eager to speak to the press, it looked in many ways like a typical, somewhat run-down district.

“Daily life in Molenbeek works well—but that’s maybe what has fooled us: that in ordinary life, there are no difficulties,” Schepmans, the mayor, said. “And next to that, there are people living in the shadow. And we have left them living in the shadow. We didn’t ask ourselves the right questions.”