The Genocide Network, a body established by the European Union in 2002 to coordinate investigators and prosecutors, based in the European Judicial Cooperation Agency Eurogist, said: "Many of the ISIS terrorist suspects are currently facing charges under domestic terrorism laws only."
"They may face broader accusations under international laws covering the worst crimes that occur during conflicts, including genocide and crimes against humanity," It said in a report carried by Al-Hurra. "and ISIS should not be considered only a terrorist organization."
It added that ISIS mercenaries could be treated under international humanitarian law "as a party to armed conflict in Iraq and Syria."
Accordingly, the body says, "Its members and foreign fighters may be responsible for war crimes and other major international crimes."
The body cited examples of returning foreign mercenaries, who are being tried on cumulative charges already in France, Germany and the Netherlands.
The Genocide Network says that "such trials can be held on a global basis to try other foreign fighters returning to their countries of origin" The trial of terrorist crimes is associated with acts of core international crimes that lead to higher penalties and provide more justice to victims.
"By recognizing these crimes and naming them as they are, justice can be done for the victims," it added.
The report quoted a study conducted by the London-based International Center for the Study of Extremism in mid-2018, saying that there were 7,252 people in Eastern Europe, and 5,904 others in Western Europe linked to ISIS.
Thousands of foreign mercenaries, who have joined the ISIS mercenaries, are still being held in detention centers in northern and eastern Syria, without trial.