From Russia With Hate

Peter Bergen and David Sterman wrote for CNN about Russian citizens that join ISIS: 

The news that the Istanbul attack was carried out by a Russian and citizens of Central Asian states that were once part of the Soviet Union might surprise those who have hitherto seen the group as a collection of mostly Arab fighters with a large Western European contingent.

Yet in fact, Russian citizens -- many of whom are Chechens or Dagestanis from the largely Muslim North Caucasus region of Russia -- are the largest group of foot soldiers in ISIS from a non-Muslim majority country, and they have played key roles in the group.

According to Turkish officials the attack at Istanbul airport was carried out by terrorists from Russia, Uzbekistan, and Kyrgyzstan and was planned by ISIS's leadership.