Russian nationals make up the largest number of jihadists from non-predominantly Muslim countries fighting on behalf of the Islamic State (ISIS/ISIL), according to data from a U.S.-based intelligence consulting firm.

Although no specific group has claimed responsibility for the Monday’s suicide attack in St. Petersburg that killed at least 14 people and wounded another 49, ISIS celebrated it on social media.

The terrorist group has established a wing in the North Caucus region, home to jihadists who have targeted Russia’s transport infrastructure on various occasions over the past two years.

According to the Soufan Group (TSG), at least 2,400 foreign fighters from Russia have traveled to engage in jihad in Iraq and Syria, the majority alongside ISIS.

The widely cited firm’s report does not account for fighters who joined ISIS after 2015 when the terrorist group established its branch in the North Caucus.

Russia is the third top country with the most foreign fighters in the region, after Muslim-majority Tunisia and Saudi Arabia, TSG reported.

As of the date when the report was issued (December 2015), experts like Soufan estimated that between 27,000 and 31,000 people from at least 86 countries had traveled to Syria and Iraq to join the Islamic State and other violent jihadist groups.

The most recent estimate from the Pentagon reveals that some 40,000 foreign terrorist fighters from at least 120 countries have fought for ISIS in Iraq and Syria.

“We know we have killed several thousands of foreign fighters in Iraq and Syria,” Army Lt. Gen. Michael K. Nagata, director of the National Counterterrorism Center’s Directorate for Strategic Operational Planning, told the Center for Strategic and International Studies on Wednesday.

“But we are unable to give you a precise number. It’s a substantial number,” he added.

Between the time Soufan released its report and the most recent U.S. government estimate, the number of estimated foreign fighters who traveled to the Middle East to engage in jihad increased by at least 9,000 or an estimated 30 percent.

In October 2015, Russian President Vladimir Putin said that 5,000 to 7,000 fighters from Russia and the former Soviet republics had traveled to Syria to join ISIS.

Some former Soviet republics are predominantly Muslim.

The Soufan Group reported a few months later that “based on the best available information, TSG calculates that there are at least 4,700 fighters from the region, in line with the lower end of the official estimate provided by Russian authorities.”

On Monday, the consulting firm wrote on Twitter, “Outside of MENA [the Middle East and North Africa], fighters from former Soviet Republics make up the second largest demographic of foreign fighters.”

Referring to the recent suicide attack in St. Petersburg, TSG added on Twitter Tuesday, “Chechen terror groups and the Islamic State each have the ability and motives for an attack.”

Nearly a year after TSG reported that 2,400 fighters from Russia had joined ISIS, Australia’s Lowy Institute placed the estimate at 2,500, noting that Russia is the top non-Muslim country with jihadists in the Middle East.

“According to open source analyses of available foreign fighter statistics, 12 countries make up at least 75 percent of the foreign fighter contingent in Syria,” it reported.

“Tunisia (with 6000–7000 fighters), Saudi Arabia, Jordan, Turkey, and Russia (around 2500 each), and France and Morocco (1500–1700 each) account for the bulk of the source countries,” added Lowy. “Indonesia, Egypt, the United Kingdom, Germany, and Lebanon are estimated to have provided between 500 and 1000 fighters each.”