According to the EUROPOL, as well as, credible media sources in Austria, Croatia, Slovenia and Bosnia there is already established infrastructure that can facilitate the expansion and attacks of the Islamic State in the EU.
In Slovenia, since the opening of the borders, several members of the so-called Islamic state entered, via the mass refugee wave. Instead of going northwards they remained in the outskirts of Ljubljana. "Comrades" from Germany, Luxembourg, Slovenia and Austria joined them and engaged altogether in shooting practices in dense forest regions near the city.
Officially Slovenian Police does not confirm neither denies the aforementioned, but it states it has no valid information regarding Jihadist training in its territory. It has to be noted that in nearby Bosnia-Herzegovina, training of various cells of aspiring Jihadists is in action for years, and takes place in remote villages, abandoned industrial facilities and mountain ranges.
Similar findings but without official affirmation have been traced in Sanjak-Raska in Serbia and in the nearby rugged mountainous territory of North -East Montenegro. In the past various attempts to forge training grounds took place in Kosovo, FYROM, Albania and South Bulgaria.
The total number of radicalized Islamists that can potentially be mobilized ad hoc , as "Lone Wolves" or via interaction with incoming Islamic State fighters across Europe can be roughly estimated at 100,000 people, although the secondary and tertiary support and logistics networks (often unaware of their involvement) are certainly much higher.
Of course the above refers to a conclusive spread out of the threat and in pure operational terms far less would be able to organize themselves effectively and strike without been apprehend just before. Still the issue has become a ticking bomb for the whole of Europe and encompasses the greatest threat for the Continent since the end of WW2.