Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is the author of the new book " United States of Jihad: Investigating America's Homegrown Terrorists ." David Sterman is a senior program associate at New America's International Security Program and holds a master's degree from Georgetown University's Center for Security Studies.

(CNN) 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.

The Soufan Group, a New York-based intelligence consulting firm that tracks "foreign fighters" who have joined ISIS, estimates that 2,400 Russians have traveled to Syria. It placed the number of fighters from Uzbekistan and Kyrgyzstan at 500 each.

In October, Russian President Vladimir Putin estimated the number of fighters who had left for Syria from Russia and the former Soviet republics at 5,000 to 7,000.

Individuals from the former Soviet republics have risen to the leadership ranks in ISIS. The most well known is Omar Shishani, killed in an American airstrike earlier this year, was an ethnic Chechen who had a $5 million U.S. reward on his head at the time of his death.

He was the group's commander in northern Syria and he also oversaw the prison in ISIS's de facto Syrian capital, Raqqa, in which the terrorist group held foreign hostages.

The significant role of Russians and Central Asians in ISIS is also confirmed by the terrorist group's own internal documents.

The Combating Terrorism Center at West Point examined ISIS files that were obtained from an ISIS defector who gave them to NBC News last year.

The ISIS files contained documentation for some 4,600 militants who had joined the group from around the world.

One-hundred and forty-one of these militants listed Russia as their country of citizenship. That makes Russia the sixth-largest national contingent joining ISIS from around the world, according to the documents analyzed by West Point.

It also gives Russia the dubious distinction of contributing the most fighters to ISIS from any non-Muslim majority country, surpassing even France, the leading European contributor of fighters to ISIS.

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The ISIS files also include records of 72 Uzbeks and 47 fighters from Kyrgyzstan.

Many of the militants joining ISIS from Russia and the former Soviet republics were involved in Islamist insurgencies in their home countries.

Chechnya has produced many of these fighters following the two Chechen Wars that began in the 1990s, pitting Russian forces against Chechen separatists.

The Russians deployed brutal tactics in Chechnya, which radicalized the insurgents and moved them in a more Islamist and militant direction.

Similarly, the Uzbek government has long waged a brutal war against Islamist insurgents in their country.

A testament to the ubiquity of the Russian fighters in ISIS was provided by Jamal Khweis , a 26-year-old from Virginia, who was captured in Iraq by Kurdish forces earlier this year after allegedly having fought for ISIS.

Khweis told his captors that during his time in ISIS guesthouses he was surrounded by Russians and Uzbeks.

ISIS caters to its potential recruits in Russia with Russian language propaganda. Last year, ISIS debuted a Russian language propaganda magazine titled Istok and the group also launched Furat Media , a Russian-language social media wing, through which it declared its establishment of an ISIS "province" in the North Caucasus area of Russia.

The best way to crack down on the threat posed by the thousands of fighters from Russia and the republics of the former Soviet Union fighting with ISIS is for those governments to give Interpol the names of those militants that they know have gone for training and fighting in Syria.

Right now Interpol only has the names of some 5,000 militants who have trained in Syria, which is only a relatively small fraction of the some 30,000 "foreign fighters" who have gone there.

The Istanbul airport attacks remind us that though ISIS is rooted in the Middle East and draws upon a contingent of alienated European Muslims, fighters from Russia and the former Soviet republics also play an important role in its self-styled jihad.