“There are parts of Europe, especially in France and Belgium, where over the past two decades you’ve seen the emergence of essentially ungoverned spaces, nearly akin to Yemen or Libya,” said Peter R. Neumann, director of the International Center for the Study of Radicalization and Political Violence at King’s College London. “Molenbeek is one of them, a place where local authorities and even mainstream Muslim groups abandoned them, with an informal pact, that ‘as long as we don’t see you, we won’t bother you.’ ”

Criminal groups, but also Islamist radicals, soon “figured out that this local anger could be channeled into radical extremism,” he said. Resentment and alienation from the state meant that these groups “could enter and work without being hassled by the police but also found people open to their message.”

Mr. Heisbourg said that the result was “a disaster for counterterrorism.” These were “no-go areas for the authorities, who have found it very difficult to get informants and human intelligence,” noting that many of the French citizens who carried out attacks in France lived or were hosted in Brussels neighborhoods like Molenbeek.

In per capita terms, more Belgians have left to fight with Islamic militants in Syria and Iraq than citizens of any other European country: As of last month, 441 had done so, according to data from the Belgian Ministry of Justice, and 117 have returned.

Places like Molenbeek and Schaerbeek, where the bombs used on Tuesday were thought to have been constructed, have been problems for a long time, Mr. Pantucci said. “One question is whether more could have been or should have been done to understand the local problems and deal with them. Because left alone, over time there is an undercurrent of radical ideologies,” radical preachers and recruitment networks, “and getting rid of that is very, very difficult.”

Some political scientists, like Olivier Roy, a scholar of Islam at the European University Institute, say that Islam does not cause radicalization, but serves as the vehicle for radicalized anger from some Muslim youth.