David Cameron calls for EU crackdown on illegal firearms - BBC News
http://www.bbc.co.uk/news/uk-politics-35086934

I think that's the first time I've ever heard about weapons coming from the Balkans; over here they came from ten radically different countries at the very least, but I never heard the Balkans mentioned. Though I did wonder where the weapons for previous French atrocities came from.

Well, most immediately Belgium, but who the hell knows originally. Nobody knows how many AK-47s actually exist. It's actually not that hard to get guns into an EU country, provided you don't want to ship lots and only want to do it once or twice.