Now, as the United States carries out a sudden and unplanned withdrawal from the country, senior officials across government say that the U.S. has no real idea how many fighters have already escaped amid the fierce fighting between Turkey and Kurdish fighters that Washington previously backed in the fight against ISIS.

“Nobody does,” a senior government official involved in the issue told Defense One.

The fear is that large-scale breakouts will allow ISIS, which survives in sleeper cells hidden among the population in Iraq and Syria, will be able to reconstitute itself with the return of experienced fighters to the battlefield. Last month, ISIS leader Abu Bakr al-Baghdadi ordered his followers to accelerate efforts to free fighters.