LTC (Res) Dr. Daniel H. Heinke, German Armed Forces Reserve, and Director, State Bureau of Investigation, Germany's Bremen Police published his research on Foreign Terrorist Fighters as a Marshall Center Insights Paper. He talks more about his research in this interview.

You can find his Insights paper at: http://www.marshallcenter.org/mcpublicweb/mcdocs/files/College/F_Publications/secInsights/security_insights_15.pdf