Belgian soldiers during tensions between police and residents in the Brussels suburb of Molenbeek in April 2016. (Yves Herman/Reuters)

BERLIN — There was a time in Europe when it was easier for terrorists to buy chemicals and build bombs than to obtain military-style firearms. Then came the November 2015 Paris attacks that served as a devastating wake-up call for how much the threat posed by terrorists, and their weapons, had changed.

Now, an international group of researchers is warning that the firearms trade that enabled militants to obtain those assault rifles is, in fact, still expanding. Militants determined to strike European targets are among the groups and individuals benefiting the most from what the researchers are describing as an “arms race.”

The study, funded by the European Commission and due to be released Wednesday as part of the Studying the Acquisition of illicit Firearms by Terrorists in Europe (SAFTE) project, warns “the increased availability of firearms has contributed to arms races between criminal groups” across the European Union.

The growing competition, the team of European researchers concludes, is posing a new challenge to European authorities who see themselves confronted with a “gradual trickling-down of the possession and use of firearms to lower segments of the criminal hierarchy in several EU member states, especially in Western Europe.” What used to be a relatively closed market has become more accessible in recent years, even though vast regional differences remain.

While illicit handguns cost between $2,300 and $3,000 in Denmark, Croatian dealers are offering similar products for 1/20th of the price. But purchasing an illicit firearm in Croatia would still be nearly impossible for a Danish criminal with no local connections. “Having the right criminal connections and reputation are crucial factors,” the group of researchers writes.

Legal firearms sales are much more tightly regulated in Europe than in the United States, so weapons are often smuggled from the western Balkans into the borderless Schengen area that includes countries such as France, Germany and Italy. Europe’s borderless area that ranges from countries such as Slovenia in the east to Portugal in the far west of the continent may be an advantage for travelers and traders, but its expansion has also eased the work of smugglers who can now access most of the continent without having to fear border checks. The conversion of blank-firing guns and reactivation of discarded weapons is also a source of illegal firearms.

The customers of weapons dealers are increasingly individuals or groups seeking to use them during attacks. While there used to be little overlap between criminal gangs and militants only 10 years ago, multiple studies have found a growing connection between both spheres. With more firearms in circulation among criminals, those with political violence on their minds have also found it easier to acquire more lethal weapons.

“Different types of criminals tend to procure, possess and use different types of firearms, and contemporary terrorist networks usually rely on established criminal connections to acquire firearms from these markets,” the researchers write.

“Prisons have also been identified as places that offer new opportunities for terrorists who do not yet have the necessary criminal connections to acquire firearms,” according to the cross-European team that included researchers from the Netherlands, Sweden and Italy.

The findings match previous studies that also observed a common pattern in Europe’s crime-terror nexus. A 2016 report by the International Center for the Study of Radicalization and Political Violence at King's College London found criminals were actively encouraged to join organizations such as the Islamic State — a break from militant groups such as al-Qaeda that sought tighter control over their ranks.

While the more recent European Commission-funded firearms study found no cases of dealers exclusively or deliberately selling weapons to terrorists — possibly because the risk of detection is higher if national security is at stake — the blurring of lines between both spheres appears to have made it harder for intelligence services to prevent weapons sales to militant groups.

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