The rise of the Islamic State of Iraq and the Levant (ISIL) in Syria and Iraq is raising fears of radicalisation across Europe.
The perception is of battle-hardened fighters returning from the frontline, full of anger and resentment, prepared to carry on the fight at home.
Numbers vary, but it is estimated around 3,000 European citizens have taken up arms in Syria.
The UK is leading European governments in drawing up measures to combat the potential threat posed by assumed radicals.
In its latest report, the European Union's law enforcement agency, Europol, categorised 152 "terrorist attacks" in EU member states in 2013.
Of those, most were related to separatist groups, while "religious radicalisation" appeared to be evident in just two attacks.
So how do authorites balance the perceived threat from a comparitively small numbers of fighters returning from abroad, against the threat from myriad groups, categorised as "extremist", already established on their soil?
Mathieu Guidere - professor of Islamic Studies at the University of Toulouse and a specialist on religious extremism.
Francesco Ragazzi - co-author of the report ‘Preventing and countering youth radicalisation in the EU'.
Mansouria Mokhefi - special adviser on the Middle East and North Africa at the French Institute for International Relations.
Live Box 201493123623889276
Source: Al Jazeera