According to the International Center for the Study of Radicalization and Political Violence at King's College London, criminals-turned-terrorist suspects are becoming a more common problem. The research center collected extensive data on 79 recent European militants with criminal pasts. About 80 percent of those involved in recent terrorism plots had criminal convictions.

The researchers fear that extremists with criminal pasts could plan large-scale attacks more easily because of underworld contacts and previous experience dodging the law. “Because of the changing profiles of terrorists, it is easier and cheaper for them to obtain firearms,” said Nils Duquet, senior weapons researcher at the Flemish Peace Institute.

The growing market for firearms has led to an increase in supply. The number of seized illicit firearms in Europe has been on the rise for years.

A research project funded by the European Commission and coordinated by the Transcrime Joint Research Centre on Transnational Crime also laid out Europe's vast weapons trafficking network. The “Fire Project” documents all firearms that were seized in the European Union between 2010 and early 2015.

One country in Western Europe stands out among the rest: Belgium. Across the country, thousands of firearms were seized over the last few years — many more than in other Western European countries.

Many of the recent major terror attacks in Europe were planned in Belgium's capital, Brussels, where police have raided dozens of homes belonging to suspected Islamic State sympathizers or other militants. Experts, however, say that only a fraction of all illicit firearms which circulate in the small European country were found.

“What authorities focus on is gaining some time,” said Duquet. “The aim is to make sure that terrorists need extra steps to obtain a gun — for instance, one week. Every additional day you win is an extra day for authorities to intervene.”

Most trafficking takes place on land routes, rather than on ships or planes. Criminals involved in the trafficking business enter the European Union's Schengen area, where there are few border checks, through E.U. members Slovenia or Hungary.

So how easy is it to cross the border into the Schengen area? A look at the chart below helps explain the answer: very easy, in most cases. So far, few E.U. citizens are thoroughly screened while entering the Schengen zone from outside. While non-E. U. citizens are frequently asked to stop and have their vehicles searched, E.U. citizens are mostly allowed to pass.

This explains why the vast majority of those involved in firearms trafficking on European Union soil are citizens of the union. Many come from Southern Europe.

Although the vast majority of weapons smugglers are young, between 20 and 34, there is also a surprising number of older criminals who earn money from the business.

The mixed profiles of those offering illicit firearms in Europe is matched by an increasingly diverse demand for the weapons on offer.

“Ten years ago you wouldn't have found Kalashnikovs in Western Europe,” said Duquet, referring to an assault rifle developed in the Soviet Union and now widely copied around the world. Tens of thousands of those weapons still circulate in the Balkans, where war still raged two decades ago. Many of those weapons are now making their way to Western Europe, often smuggled in small quantities.

“For criminals, Kalashnikovs are not that interesting. You can't really rob a store with it,” said Duquet. “But for terrorists, Kalashnikovs are ideal. They kill many people in a short time frame.”

You can explore the data yourself on Project Fire's interactive website.