I have refrained from posting the many reports on the flow of European recruits to the violent jihad in Syria, with the various estimates of the numbers involved and the rare report on persons returning being arrested.

Here is a good introduction:
Syria since 2011 has emerged as the greatest of all jihad contests for foreign fighters. Given its proximity to Europe, large numbers of Westerners have gone to fight in Syria, via Turkish “ratlines,” raising concerns among European security services about what these violent young men might do when they return home. Significant numbers of angry young mujahidin from Europe have joined the fight in Syria, on a scale never before seen in counterterrorism circles, leading to something approaching panic among Western intelligence agencies.
Link:http://20committee.com/2013/11/16/sy...eaches-europe/

The article is about recent arrests in Kosovo, the author wonders if the action was prompted by militants assaulting two US citizens.