The industrious Norwegian SME Thomas Hegghammer was interviewed on what kinds of terror attacks do we have to expect in Europe, and how dangerous are returning Foreign Fighters? Link:http://abususu.blogspot.de/2015/01/t...us-for-at.html

Here is one passage:
....we do know is the proportion of people who returned from previous battlefields and then plotted attacks. Before Syria, that rate was 1 out of 15 to 20. If you look at open source data about returnees from Syria who were involved in terror plots across Europe, we have so far seen about 10 plots with roughly 20 returnees involved. That is 20 out of 3000 who left to fight abroad, or 20 out of just over 1000 who have already returned, repectively. So far, it is only a small minority who have become terrorists. The question before us is: How do you stop that minority without over-reacting towards the relatively harmless majority?


Yes there is a thread on foreign fighters, but IMHO what Hegghammer says has few equals.