Brussels itself is about 25 percent Muslim — 70 percent are of Moroccan heritage and 20 percent Turkish, and the ethnic groups tend to stick to themselves, making them difficult for outsiders, like the police, to penetrate.

Belgium’s integration has been somewhere between the French model, which put new immigrants in suburban ghettos, and the British and American one, which created communities like Chinatown or Little Italy, Mr. Goldstein said. “In Brussels, everyone lives in the city, and we chose a model of diversity through mixing of populations in the same neighborhoods.”

But “we failed,” he said. “We failed in Molenbeek” and Schaerbeek, too, to ensure the mixing of populations.

“We have neighborhoods where people only see the same people, go to school with the same people,” he said. “What connection do they have with the whole society, what connection do they have with real diversity? It’s the establishment of the ghetto,” he said, “and it’s the thing in our urban development that we have to tackle.”

Jews have left Schaerbeek, and the last two synagogues are being sold. Instead, there is a kind of suffocating, insular, ethnic uniformity. “These young people will never go to museums until 18 or 20 — they never saw Chagall, they never saw Dalí, they never saw Warhol, they don’t know what it is to dream,” Mr. Goldstein said.

Of course there is poverty and unemployment, he said. “But we don’t give these young people the keys to think differently, to think outside the little box, the little neighborhood where they live — this ideological box, this closed-eyes box.”