At least one ex con from the US basically a volunteer mercenary has a criminal record and/or is/was a sex offender. He served his time and has no open cases against him. The politics of no boots on the ground makes this situation tricky along with the use of any mercenary or volunteer from the US.
http://www.thedailybeast.com/articl...-fighting-isis-is-convicted-sex-offender.html
http://www.thedailybeast.com/articl...-fighting-isis-is-convicted-sex-offender.html