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.”