Lars Bové, the author of a book on the Belgian security system, said that cooperation between different layers of government and different security services was improving but that information sharing remained a problem, particularly between federal agencies and local authorities.

Responsibilities, he said, “tend to overlap,” with only fuzzy rules for who is supposed to do what.

Muriel Targnion, the mayor of the eastern town of Verviers, where the federal police stormed a terrorist safe house in January, said she had been told by security services in Brussels that her town had 34 residents suspected as jihadists. But that was all she was allowed to know.

“All I was given was a number,” she said. “No names, no addresses. Nothing.”

History of Rivalries

Information sharing does not come easy in a country with fierce rivalries between groups that, in some cases, cannot talk to each other, at least not in a common language.

On top of language, said Sus van Elzen, a Flemish writer and former political magazine editor, “it is in our genes to reject all centralizing power” and, on all sides of the linguistic divide, to mistrust outsiders.Belgium’s history, he added, is a “very unhappy story” of constant retreat from intruding forces, notably the Spanish, the French and the Germans, that have sought to impose a centralized order.

Belgium was formed from part of what were known as the Low Countries, which for centuries were fought over by the dukes of Burgundy, Hapsburg emperors and the rulers of France. The chief languages, Dutch and French, became instruments of those in power, and both fell in and out of favor. After the defeat of Napoleon in 1815, the Congress of Vienna made the largely Roman Catholic Belgium part of Holland, which was ruled by a Protestant king. The union led to unrest, and eventually rebellion, and in 1831 another gathering of Europe’s great powers established the kingdom of Belgium.

“Everything in Belgium is politicized; you cannot have an administrative function, particularly a senior one, if you don’t have a political affiliation,” said Claude Moniquet, a former French intelligence officer who runs a risk analysis company in Brussels.

Language Divisions

Language divisions might not prevent intelligence experts from communicating, but they shape the political environment in which the experts operate and decide who fills the ministerial positions that set their priorities.

Mr. Jambon, the Flemish nationalist interior minister, has infuriated many French-speaking Belgians with what they see as insinuation that they alone are to blame for the growth of Islamic militancy in Belgium.

“The link that has again been established between terrorism and our country forces us to look into the mirror,” Mr. Jambon said two days after the Paris attacks. The reflection he saw, however, was heavily filtered by the lens of Belgium’s tribal politics.

“The question I ask myself is: Why did we succeed to eradicate radicalism in Antwerp and other Flemish cities, and why doesn’t it work in Brussels?” he said, contrasting his Dutch-speaking region of Flanders with Belgium’s mostly French-speaking capital.

Antwerp, the largest city in Flanders, has cracked down on Islamic extremists. This year the city was the site of Belgium’s biggest terrorism trial, with more than 40 defendants accused of traveling to fight in Syria or of encouraging others to do so.

But the trial was held in Dutch-speaking Antwerp only because that was where its principal defendant, Fouad Belkacem, and the now-banned organization he led, Sharia4Belgium, a well-known recruiter for jihad, had operated for years.

From there, Mr. Belkacem reached out to French-speaking areas, notably Molenbeek, where in 2012 he organized a rally outside a police station to protest the arrest of a woman wearing an Islamic head covering.

Before the trial, when he was sentenced to 12 years for supporting terrorism, Mr. Belkacem was “very active here,” Ms. Schepmans said, and he set alarm bells ringing about the dangers of extremism.

The federal authorities were so worried that they offered to help Molenbeek with money to set up a unit to combat radicalization. But the money offered was paltry — initially 40,000 euros, or about $42,500, and later €60,000. The borough found funding from its own budget and the radicalization unit now has four employees. Only one speaks Arabic.

Molenbeek’s police force, housed in a big concrete block adjoining City Hall, knows the neighborhood and its residents, but the mayor said that it “has neither the means nor the powers” to keep tabs on Islamic militant suspects.

Arthur van Amerongen, a Dutch writer on the Middle East who lived in Molenbeek a decade ago while doing research for a book on Islamic extremism, said it had been obvious for years, particularly under a Socialist mayor who governed until 2012, that militants were making inroads there, but “nobody wanted to know because this did not fit their political agenda.”

Intelligence services, too, have struggled with the same political calculations and constraints.

While long derided for its often chaotic and fractious ways, Belgium is if anything “overorganized,” with so many overlapping bodies and agencies that nobody is ever really in charge, said Hubert van Humbeeck, a Belgian political commentator. “It works more or less normally, but when something so unpredictable like terrorism happens, all the institutions collide.”

“This is the Belgium disease,” he added. “Everyone always says it is not their fault, and they are often right.”