In France individuals, except for certified collectors, are prohibited from owning military-class arms. Those hoping to possess a handgun or hunting rifle in France must pass a stringent background check and a mental health evaluation and must obtain a license.

While semi- and fully-automatic firearms are illegal in France, they have become increasingly common in recent years. The Paris-based National Observatory for Delinquency, a government body created in 2003, reports that the number of illegal weapons in France has been steadily increasing by double-digit percentages for the last several years.

In October 2014, raids of several homes in four corners of France revealed stashes of hundreds of weapons, including assault rifles, machine guns and automatic pistols. Forty-eight people were arrested in what was discovered to be a major Internet weapon-trafficking ring, with its headquarters thought to be in Paris.

Most of the arms found on the black market in France and other Western European countries hail from countries, many in Eastern Europe, with a proliferation of guns and loose or poorly enforced regulations or are trafficked from conflict zones, according to experts.

“One of the reasons we see a lot of Kalashnikovs and AK-47s on the black market is because Russia has just upgraded the Kalashnikov, and that has created massive stockpiles of the older models,” said Kathie Lynn Austin, an expert on arms trafficking and the director of the Conflict Awareness Project, an international nongovernmental organization dedicated to investigating major arms traffickers.

The AK-47s used in the Charlie Hebdo attacks were reportedly purchased legally, traced back to a shop in Slovakia that sells decommissioned weapons as props for movies, a Slovakian newspaper reported in February.

The guns were reconverted to fire live ammunition and were eventually sold to the Paris attackers through a Belgian arms dealer in Brussels, who also sold them a rocket-propelled grenade launcher, Haaretz reported.

Weapons often flow to Western Europe from the Balkan region, which includes Albania, Bosnia and Herzegovina, Croatia, Kosovo, Montenegro, Serbia, and Macedonia. The Geneva-based research group Small Arms Survey estimates that, in the region, between 500,000 and 1.6 million households own firearms.

Weapons left over from wars are also often trafficked across borders, which was the case after the Libyan revolution in 2011. The New York–based nonprofit Foreign Policy Association says that there are millions of weapons from the conflict and there is little regulation by government officials.

“There are definitely concerns of these weapons making their way into Western Europe,” says Glenn McDonald, a senior researcher at Small Arms Survey.

Austin said some governments enter into what she calls the gray market trade, in which arms sold legally can end up on the black market. “This involves weapons that originate from a legal government transfer,” she said, “but when they arm proxy forces to carry out their national security agendas, they may use illicit channels or black market arms brokers to carry out the deal.”

There are approximately 1,000 manufacturers of legal weapons in nearly 100 countries. The governments of the five permanent members of the U.N. Security Council — Russia, China, the U.S., the U.K. and France — are the main exporters, and Western European nations like Germany and Belgium also export significant stocks.

Experts say arms brokers play a primary role in black market trafficking, often forging government documents or licenses to complete deals.

France’s government tightened its regulations on illegal arms possession in 2012, approving longer jail terms and bigger fines for those in possession of illicit weapons after Mohamed Merah shot and killed seven people in Toulouse. But it’s unclear how stringently the laws have been enforced.

According to McDonald, Europe has been hesitant to conform to stricter directives when it comes to the illicit arms trade because of the red tape and cost involved. But the latest string of attacks are likely to change that, and the European Union has moved toward putting in place stronger regulations in recent years.

In 2013, Interpol created a centralized tracking system, the Illicit Arms Records and Tracing Management System (iARMS), for Interpol member countries to report lost, stolen, trafficked and smuggled firearms. And Europol, the EU’s law enforcement agency, said in a communications document that year that disrupting illicit manufacturing and trafficking in firearms would be one of its priorities for 2014 through 2017.

The initiative includes looking at the rise in e-trafficking, the sale and purchase of illicit firearms over the Internet.

Many final purchases of illegal arms take place over the Internet, with payment methods in place like bitcoin that make it almost impossible to trace buyers. With a few clicks, users can purchase an AK-47 for less than $2,000.