Riot police armed with machine guns controls the street in front of Pasing railway station on Jan. 1, 2016 in Munich, Germany. Getty Images

German authorities Tuesday carried out 13 raids throughout five states targeting terror financing rings, Australian Associated Press reported. The searches were conducted in 12 homes and an accommodating facility for refugees located in Thuringia, Saxony, Bavaria, North Rhine-Westphalia and Hamburg.

The raids marked the latest attempt by a European country to counter threats made against targets in the West by global terror group Islamic State. The militant organization, also known as ISIS, has managed to attract followers and fighters from across the world, including many countries in Europe. Foreign fighters have swelled the group’s ranks in Iraq and Syria, allowing it to take over massive amounts of territory and enforce its self-proclaimed caliphate.

While the group has since lost much of its territory, concerns are growing as to what these fighters will do once they return home. ISIS attacks in France and Belgium have already killed hundreds and numerous ISIS-inspired attacks have occurred throughout Europe since 2014.

E.U. Security Commissioner Julian King warned last week in an interview with Die Welt that the ongoing Iraqi offensive to clear ISIS out of its stronghold in Mosul could prompt ISIS fighters to begin returning home to Europe. Though he dismissed the expectation of a “mass exodus,” he maintained that even “a small number [returning] poses a serious threat.”

Fear of returning ISIS fighters has also shaped how nations treat migrants fleeing Syra and other war zones, with some European leaders calling for tighter border restrictions in fear that returning fighters are planning domestic attacks.

The European country with the most ISIS fighters per million people is Belgium, according to 2015 statistics by the International Center for the Study of Radicalization and Political Violence. With an estimated 520 militants, Belgium’s ISIS population is 46 per million. Next on the list is Sweden. Sweden is believed to have somewhere between 250 and 300 militants fighting abroad, or about 32 fighters per million people. Following Sweden is Denmark.

While France finds itself fourth on the list with 18 ISIS fighters per million people, its larger population means that its claim to some 1,600 militants accounts for 32 percent of all 5,000 European militants estimated to be fighting abroad, the most of any Western country.

Russia has the most ISIS fighters overall in Europe, with some 2,700 militants.