The U.S.-backed Syrian Democratic Forces (SDF) said in a statement the prison had been brought under control by Monday morning and that all the prisoners were accounted for. But an SDF official in the area later said that about 1,000 prisoners were still on the loose inside the facility after they broke down prison doors and internal walls.

Kurdish officials said earlier they were also hunting for an unknown number of detainees who managed to escape in what appears to have been the most serious attempted breakout yet by the thousands of former Islamic State fighters detained after their military defeat last year.

The revolt took place at a prison in the city of Hasaka that houses about 5,000 Islamic State fighters of multiple nationalities who were captured after the group’s final stand in the village of Baghouz, Kurdish officials and local journalists said.

Read more: Washington Post