Belgian police arrested three people on Saturday in raids in a poor, immigrant quarter of Brussels as they pursued emerging links between the Paris attacks and an Islamist bastion in France's northern neighbor.
Prime Minister Charles Michel said at least one of those held from the inner Brussels neighborhood of Molenbeek was believed to have spent the previous evening in Paris, where two cars registered in Belgium were impounded close to scenes of some of the violence, including the Bataclan music hall.
"Police operations will go on," Michel told RTL television as local media reported continued security activity overnight in Molenbeek, west of the city centre, which is home to many Muslims, notably families originally from Morocco and Turkey.
A French prosecutor said a car hired in Belgium was linked to the attacks and that a Frenchman living in Brussels rented it and was later stopped early on Saturday at the Belgian border.
Proportional to its 11 million population, of whom half a million are Muslim, Belgium has been the European country which has contributed the most foreign fighters to the civil war in Syria -- over 300 by official estimates a year ago -- and it has figured in many Islamist attacks and plots across the continent.
Molenbeek has been connected with two attacks in France this year alone. Security officials have said the Islamist who killed people at a Paris kosher grocery in January at the time of the attack on the magazine Charlie Hebdo acquired weapons in the district. So too did the man overpowered in August on a Thalys high-speed train from Brussels to Paris before he killed anyone.
An alleged plot to attack Belgian police stations in January, which was broken up by raids in which two men were killed in the eastern town of Verviers, had connections to Molenbeek. And a Frenchman accused of shooting dead four people last year at the Jewish Museum in the Belgian capital also spent time in the area.
One international security expert said, "Belgium is a bit the Achilles heel of Europe." Another, Rafaello Pantucci of London's Royal United Services Institut, said: "The networks between France and Belgium have been very tight for some time."
Michel said: "Belgium has a central position at the heart of Europe, a small country whose local scale favours the movement of people with hostile intent," he said. But he insisted his centre-right coalition, in power for a year, was tackling that.
Belgian police officers patrol at Gare du Midi/Zuidstation railway station in Brussels, Belgium, November 14, 2015, after the attacks in Paris on Friday. Photo: Reuters/Eric Vidal
The problem of young men returning radicalized, and with skills in handling automatic weapons remained, however, Michel, 39, said: "It's always possible to slip through the net."
His interior minister, Jan Jambon, told Belgian television he believed Brussels and Molenbeek in particular was a problem and that he would personally take charge of sorting out issues in a neighborhood which conservative critics view as an example of failed left-wing experiments in mass immigration.
The district's mayor said many radicals in Molenbeek were just passing through, taking advantage of the anonymity afforded by areas of the borough almost entirely populated by Muslims.
The Belgian prosecutors would not say whether any of those arrested on Saturday were previously known to authorities.
At least three of those killed in the Paris attacks were Belgian and the country launched its own anti-terrorist investigation into the events as a result. Michel said it would work in close cooperation with the French inquiries.
After a meeting of the national security cabinet, the government raised the level of alert across the country for large events, giving officials the ability to call in troops.
Michel urged Belgians not to travel to Paris unless absolutely necessary. Security checks were stepped up at the French border and at airports and rail stations.
Though there has been violence in Belgium, and Brussels is home to the institutions of the European Union, France, and Paris in particular, has appeared a higher-profile target, more clearly associated with the action in the war in Syria. And, across borders normally unchecked, Paris lies barely three hours by road and under 90 minutes by high-speed train from Brussels.