Those links were hammered home just last week, when Belgian authorities finally captured terrorism suspect Salah Abdeslam in Brussels's predominantly Muslim Molenbeek quarter. Abdeslam, 26, was the last known surviving participant in November's attacks in Paris, whicht left 130 people dead and were later claimed by the Islamic State. Abdeslam is a French citizen of Moroccan descent, but he was born in Brussels and later lived in Molenbeek with his family — including a 31-year-old brother, Brahim, who blew himself up in the Paris attacks.

It has been known for months that Abdeslam traveled back to Belgium after the attacks, but it was only in the past few weeks that Belgian authorities got a lead and captured him and an alleged accomplice. While the capture of Abdeslam was touted as a success, it also appeared to show that the number of people involved in the Paris attacks could be far larger than first thought. And, worryingly, there were signs that Abdeslam and the network around him were planning more attacks.

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At present, it remains unclear whether there is any link between the French terrorist suspect and the attacks on Tuesday, but it's not hard to see why many people believe there would be.

In the wake of the Paris attacks, it quickly emerged that the attackers' suspected ringleader, Abdelhamid Abaaoud, was a Belgian citizen. Abaaoud was killed in a raid in a Paris suburb five days after the attacks. Brussels was on lockdown for days after it was revealed that Abdeslam had slipped unnoticed through the French border just hours after the Paris attacks; even after the lockdown was eased, Belgian Prime Minister Charles Michel warned that the threat of an attack was “serious and imminent.” Before the raid that netted Abdeslam, a number of other raids uncovered suspected militants.

Much of the attention in the aftermath of the Paris attacks focused on French problems such as disenfranchisement and segregation in suburbs and radicalization in the prison system. However, it soon became clear that Belgium may suffer from even worse problems.

Molenbeek, an area of northwest Brussels that is home to about 100,000 people, has emerged as a particular area of concern. “There is almost always a link with Molenbeek," Michel said in November. "That’s a gigantic problem, of course."

The area, just across the canal not far from some of Brussels's more fashionable areas, first began to fill up with Turkish and Moroccan immigrants about 50 years ago. But while the area has undergone plenty of gentrification in recent years, it remains a contrast with more affluent areas of the city: Unemployment has been estimated at as much as 40 percent, and the area is full of seedy and rundown shops.

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Often those from immigrant backgrounds find themselves at a competitive disadvantage in the job market, as they speak only French and Arabic and many jobs in the city require a knowledge of French, Flemish or Dutch, and sometimes English. A growing right-wing political movement in Belgium has led to feelings of division in the country: Some Muslims say a 2012 ban on Islamic veils in public spaces is a sign of their community's alienation from the Catholic mainstream.

Molenbeek's links to radicalized groups have long been known.

“It doesn’t surprise me, because radical and political Islam in Belgium is something that grew up through the years," Bilal Benyaich, a senior fellow at a think tank called the Itinera Institute, told The Washington Post's Steven Mufson last year.

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Benyaich pointed to the arrival of funding from Saudi Arabia and other wealthy Persian Gulf states in the 1970s that was used to set up conservative religious schools in the area. A decade ago, Belgian journalist Hind Fraihi went undercover in Molenbeek and wrote a popular newspaper series that showed that disillusioned young Muslims were being influenced by radical preachers.

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Despite this, the Belgian government did not act, Fraihi told The Post last year, meaning that “there is a whole generation waiting to participate in these actions."

With the rise of the Islamic State, these ambitions found an outlet. Almost 500 Belgian citizens have traveled to Syria and Iraq amid the ongoing conflicts there, and most end up fighting on the side of the Islamic State, making Belgium the biggest known exporter of foreign Islamic State fighters in Europe. A group called Sharia4Belgium, led by a charismatic preacher called Fouad Belkacem, has been accused of being at the center of attempts to recruit foreign fighters. Other potential recruits, weary of life in Europe, have made their own links to the group online.

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While most of these fighters either remain in Syria and Iraq or have died in the fighting, others are known to have returned to Europe. Authorities believe that about 100 may have returned, including Abaaoud, the Belgian thought to have been the ringleader in the Paris attacks.

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The threat posed by these fighters and others who sympathize with the Islamic State's cause has proven difficult for Belgian authorities to contain. The problem isn't just the sheer numbers of potential jihadists: Belgium is also known as a regional hub for gun smugglers, and the country's bilingual government and culture have created problems for investigators.

“Belgium is a federal state, and that’s always an advantage for terrorists," Edwin Bakker, professor at the Center for Terrorism and Counterterrorism at the University of Leiden in the Netherlands, told Reuters in November. “Having several layers of government hampers the flow of information between investigators.”

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There had been hope that Brussels could put its links to terrorism behind it. A recent tourism campaign for the city asked interested foreigners to speak on the phone — at random — to residents, most of whom were happy to talk about the city's benefits. One of the phone booths that foreigners could call was even in Molenbeek. However, Tuesday's attacks demonstrate that terrorism remains a real and potent challenge to Belgium.