Updated | The U.S. military allowed thousands of Islamic State militant group (ISIS) fighters to flee from their de facto capital of Raqqa, Syria, in a secret deal that boosted the U.S. fight against the forces of Syrian President Bashar al-Assad, according to a former Pentagon-backed commander, who has since switched his allegiance to Turkey and who spoke to Reuters.

As the spokesperson for the Syrian Democratic Forces, a mostly Kurdish coalition of Arabs and ethnic minorities, Brigadier General Talal Silo acted as the face and the voice of the U.S.’s leading partner against ISIS in Syria. The U.S.-backed group successfully ousted ISIS from Raqqa in October. Weeks later, in mid-November, Silo handed himself over to Turkey, an enemy of Kurdish efforts in northern Syria. For the first time since his switch, the senior commander has spoken out and claimed that the U.S.-led coalition let significantly more fighters out of the embattled city than it previously admitted to.

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“[An] agreement was reached for the terrorists to leave, about 4,000 people, them and their families,” Silo told Reuters Friday, claiming that all but about 500 who fled were ISIS fighters.