WITH its stout brick houses and wrought-iron balconies, as well as occasional tower blocks, Molenbeek would not be out of place in a multicultural corner of any western European capital. Residents can shop at the Marrakech baker, Islamic butcher, Halal Turkish Pizzas or the King of Saree. A rundown Islamic centre offers classes in rapid memorisation of the Koran around the corner from a main commercial street strung with municipal Christmas lights.

In recent days, after the carnage in Paris, Molenbeek has changed in the public imagination from an edgy ethnic corner of Brussels into a nest of European jihadism. One of the Paris suicide-bombers, Brahim Abdeslam, was a French citizen who lived in the area. His younger brother, Salah, is believed to have fled back to Belgium after the carnage. Raids by armed Belgian police in Molenbeek on November 16th and 19th were intended to track him down.

The presumed mastermind of the attacks, Abdelhamid Abaaoud, a Belgian of Moroccan descent, also grew up in Molenbeek. He had been assumed to be in Syria—Islamic State’s glossy magazine even “interviewed” him about his exploits in evading capture. But on November 18th French police fought their way into a flat in Paris where he was thought to be hiding. A woman blew herself up; Mr Abaaoud died, too. The public prosecutor said the raid had foiled another imminent plot to attack the Paris region.

Molenbeek has a longer association with jihadism. Former residents of north African extraction include the killers of Ahmad Shah Massoud, the commander of Afghanistan’s anti-Taliban Northern Alliance, who was assassinated in 2001 as a prelude to al-Qaeda’s attacks on America on September 11th; a man involved in the Madrid train bombings in 2004; a man who killed four people at a Jewish museum in Brussels in 2014; and a man who tried to kill passengers on a high-speed train from Amsterdam to Paris in August. As Charles Michel, the Belgian prime minister, puts it, “Almost every time [there is an attack], there’s a link with Molenbeek.” The question of why some of Europe’s young Muslims become radicalised, fight in Syria and kill their own fellow-citizens is a conundrum that worries all governments—none more so than Belgium’s. France has more Muslim citizens than any other country in the European Union, and the largest number of foreign fighters in Syria; but Belgium has the highest proportion of those fighters as a share of its population (see chart 2). The causes are not only, and perhaps not mainly, Islamic puritanism and economic marginalisation—although Molenbeek, where locals complain of unemployment and discrimination, has its share of both. In June a French parliamentary report pointed to more personal factors: “an existential quest” for identity and belonging undertaken by those with “psychological and social malaise”. Those heading for Syria are often petty criminals. But there are also middle-class youngsters, young girls and converts, says Dounia Bouzar, who runs a French deradicalisation centre.

Despite this variety, the fact that many of the attacks in Europe in recent years have been carried out by people tagged by security agencies as potential extremists suggests the spooks are at least looking in the right pool of suspects. But the numbers are so large that it is impossible even for the most generously funded agencies to monitor them all. It apparently takes from 20 to 60 people to follow a single suspect around the clock.

Whether or not Belgium has a worse problem with radicalism than elsewhere, it is clearly struggling to cope with it. Its police and intelligence forces are, like most of the country’s institutions, fragmented and under-resourced. It has long had a reputation as the way-station for drug- and gun-smuggling between the Netherlands and France. Another problem is that Belgium lies at the heart of the Schengen zone area. Once a gun is smuggled across the external border of the free-travel area, typically from the Balkans, where they are plentiful, it can be taken freely across much of Europe. Black-market prices suggest that automatic weapons are cheapest in the Balkans and most expensive in Britain, which is outside Schengen. Some reports say that weapons used in the Charlie Hebdo attacks in Paris in January came from a specialist in Slovakia.

If the Schengen zone is border-free for holidaymakers and terrorists, it is full of unseen barriers for police and intelligence agencies. Authorities have no means of monitoring passenger-name records for cross-border travellers, even those coming from outside Schengen. Where passports are looked at, eg, at airports, the identities of Schengen-area citizens are not systematically checked against databases of suspected criminals or terrorists. Too many of the supposed joint European databases contain little data, or cannot be properly searched. Even when a suspect’s name records a hit, all that comes back is the name and number of a police officer to be contacted for information. Despite fears that jihadists could be slipping into Europe with the flood of refugees, European police forces cannot gain access to the Eurodac database that records fingerprints and other details of asylum-seekers.

Some, especially on the far right, are clamouring for the abolition of Schengen. But even if European voters were to put up with border controls, it would be impossible to check the roughly 200 road crossings between Belgium and France alone. Mr Abdeslam is said to have slipped past several police checkpoints as he fled Paris.