Two reports on returning foreign fighters, the first from a Belgian think tank, published as an Egrmont Paper covers Belgium, Netherlands and Germany (79 pgs.).

Summary:
Some 5000 men, women and children have travelled from Europe to Syria and Iraq since 2012. An estimated 1500 of these foreign terrorist fighters (FTF) have returned so far. Belgium, Germany and the Netherlands represent a third of European FTF and returnees. This report looks into the evolution of policies on returning foreign fighters in these three countries, comparing responses with regard to fighters that are still in the conflict zone, policies to deal with returnees in prison and attitudes towards the children of foreign fighters. It is the very first systematic and in-depth study into national approaches and policies vis--vis returnees. Its added value lies in the wealth of data, including data that has not been published before, and in the comparative angle.
Link:http://www.egmontinstitute.be/return...e-netherlands/

The second report (116 pgs.) is from the GW Program on Extremism 'The Traveler: American Jihadists in Syria and Iraq' and the summary refers to:
Hundreds of Americans have been drawn to jihadist organizations fighting in Syria and Iraq. Many were arrested while attempting to make the journey. The 64 individuals identified in this study all reached their destinations. This study, released in February 2018, sheds light on the motivations, methods, and threats posed by these travelers.
Link:https://extremism.gwu.edu/travelers