The answer: Eastern Europe, most likely, where the trafficking of deadly small arms is big, shady business. And where local authorities find it difficult to intervene.

The French government and the European Union know they have a foreign gun problem. But as the chain of attacks illustrates, efforts to tamp down on the flow of weapons have, so far, failed to disarm terrorists.

French police reportedly seized more than 1,500 illegal weapons in 2009 and no fewer than 2,700 in 2010. The number of illegal guns in France has swollen by double-digit percentages annually for several years, Al Jazeera reported, citing figures from Paris-based National Observatory for Delinquency.

The seizures likely made just a tiny dent in the pool of available weapons. “The fact that a Kalashnikov or a rocket launcher can be acquired for as little as 300 to 700 Euros in some parts of the EU indicates their ready availability for [organized crime groups], street gangs or groups orchestrating high-profile attacks resulting in significant numbers of casualties,” Europol, the EU’s law-enforcement agency, explained in a policy brief.