ZAGREB — The firepower displayed in Friday's attacks in Paris, including multiple explosive devices and at least five Kalashnikovs, raises the inevitable question: Where do the terrorists' weapons come from?

Terror investigators and crime experts say the most likely source are Balkan countries, a vestige of the wars that consumed the region through much of the 1990s, Croatian news website Net.Hr reports

Often large caches of weapons from the wars of the past have remained hidden in basements, attics and storages. According to a study by the Swiss organization Small Arms Survey, up to six million weapons were left behind in such countries as Serbia, Bosnia and Herzegovina, Croatia and Kosovo.

France and the European Union are well aware of this scourge, but have so far largely been unable to stem the flow of weapons into Western Europe. On the black market, experts say a Kalashnikov or even a rocket launcher can go for as little as 300 to 700 euros, which means that street gangs, criminals and terrorists can have relatively easy access to such weapons.

In 2014, Europol reported that a truck stopped with "large quantities of weapons and bombs" at the border of Slovakia. The truck came from Bosnia and Herzegovina and was on its way to Sweden.

The lesson is a grim one: Even wars of the past can provide fuel for today's conflicts. Indeed, even if smugglers run out of weapons in the Balkans, they can turn to Russia, which has been introducing new weapons into its military ranks. And who's watching over all its old Kalashnikovs?