U.S. officials have long feared that this is a natural and expected result of the long-planned military operation against thousands of ISIS fighters operating out of Iraq’s second-largest city, some of whom have already used chemical weapons.

“It’s a quite legitimate concern,” Seamus Hughes, a former U.S. National Counterterrorism Center official, told NBC News.

“We’ve seen in past conflicts, in places such as Bosnia or Afghanistan, that foreign fighters return to their home countries and use the skills and connections they acquired to plan future attacks,” said Hughes, who is now deputy director of the Program on Extremism at George Washington University. “Some undoubtedly will move on with their lives but there is, of course, a rate of recidivism and true believers that will continue on the fight. So while the short-term gains of the fall of Mosul are important, we can’t lose sight of the long-term implications when they disperse.”