Hundreds of western fighters who traveled to Iraq or Syria to fight for the Islamic State have returned home, a new report from The Soufan Center reveals.

The majority of foreign fighters within ISIS came from Russia and the Middle East, however, hundreds of them have still come back to the European Union. These include nearly half of departed fighters from the U.K., nearly 300 back to France, more than 100 in Belgium, and dozens others across Western Europe.

Returned fighters from ISIS remain a major counter-terrorism threat for western nations whose security services are already stretched thin. The report from the nonprofit organization classified the fighters into five different categories:

(i) those who left early or after only a short stay and were never particularly integrated with IS; (ii) those who stayed longer, but did not agree with everything that IS was doing; (iii) those who had no qualms about their BEYOND THE CALIPHATE 19 role or IS tactics and strategy, but decided to move on; (iv) those who were fully committed to IS but forced out by circumstances, such as the loss of territory, or were captured and sent to their home countries; and (v) those who were sent abroad by IS to fight for the caliphate elsewhere.

The fifth category “will also be the most vicious and determined of the returnees,” the report warns.

The U.S. Department of State similarly estimated in late July that nearly 30 percent of European foreign fighters for ISIS have returned to the continent. Experts fear that some of the returning foreign fighters will pursue domestic terror plots in their home countries, or even use their passports to travel elsewhere in the West.

The November 2015 Paris attacks and the March 2016 Brussels attacks both featured fighters that had traveled to the Syrian battlefield for instruction and returned to carry out complex terror plots. Europe is particularly vulnerable to foreign fighter plots because of the freedom of movement within the European Union for member state citizens.

Many EU citizens are also eligible to enter the U.S. without scrutiny for 90 days under the Visa Waiver Program.

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