Russian security officials said that around 5,000 Russians had been fighting for Islamic State militants. It was earlier reported that a Moscow State University student had fled the country to join ISIS forces.

Alexander Bortnikov, the director of the Russian Federal Security Service, said in February that around 2,000 Russian citizens were fighting for ISIS forces. Those people apparently had no trouble crossing the border to join the militants.

The revelation comes after Varvara Karaulova’s case surprised many in Russian and alarmed Interpol and Turkish security. Karaulova as well as 13 other Russians were arrested near Turkey's border with Syria.

Andrei Novikov, the head of the CIS Anti-Terrorist Center, said some of the information about Russian citizens joining ISIS forces had been “confirmed and documented.” 

Karaulova, who was arrested on June 11, had traveled to Istanbul in May with a one-way ticket. The Moscow Times reported that the 19-year-old philosophy student followed the route often used by radicalized Russians to join Middle Eastern terrorist organization.

Russian-speaking radicals appear to be active on social media. They reportedly recruit Muslim converts and guide them with travel plans. They also post love letters from extremists to their wives.

Fatima Dzhamalova, a St. Petersburg State Pediatric University student, fled the country as well to join ISIS forces. However, the young woman changed her mind after reaching Turkey. She sent text messages to her family that she regretted the decision and asked for help.

British Home Secretary Theresa May advised radicalized people against traveling. She said at a police counterterrorism conference in London Thursday that ISIS now poses the deadliest threat.

“They want to lure young women and families with false promises of starting a new life,” The Guardian quoted May. It was earlier reported that three UK sisters had traveled to Syria with their nine children.