Around 30 percent of the about 4,000 Europeans who have traveled to Syria and Iraq as so-called foreign fighters have returned to their home countries, posing a “potential security threat,” the International Centre for Counter-Terrorism said.

Most of the Europeans traveling to the Mideast war zones come from Belgium, France, Germany, and the U.K., the ICCT, an independent research organization based in The Hague, said in a report published on Friday. The highest percentage of foreign fighters per capita comes from Belgium. The report was prepared for the Dutch National Coordinator for Security and Counter-Terrorism.

“Around 2,838 of the estimated 3,922 to 4,294 foreign fighters from member states of the European Union come from” those four countries, according to the report. “The radicalization process of foreign fighters is reported to be short and often involves circles of friends radicalizing as a group and deciding to leave jointly for Syria and Iraq,” the ICCT said.

The report comes 10 days after terrorist attacks in Brussels killed more than 30 people. The suicide bombings occurred soon after Belgian police caught Salah Abdeslam, the sole surviving suspect in the Nov. 13 attacks in Paris that left 130 people dead.

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