Armed men freed around 900 prisoners from a jail in Beni, east Democratic Republic of Congo, in a coordinated attack in the early hours of Tuesday morning, the town’s mayor said, blaming an Islamist militant group operating in the area.

Only 100 out of more than 1,000 inmates remained following the simultaneous assault on the Kangbayi central prison and on the military camp defending it, said Modeste Bakwanamaha.

“Unfortunately, the attackers, who came in large numbers, managed to break the door with electrical equipment,” Bakwanamaha told Reuters by telephone. “We believe that it was the ADF who did this.”

The Allied Democratic Forces (ADF), a Ugandan armed group active in eastern Congo since the 1990s, has killed more than 1,000civilians since the start of 2019, according to U.N. figures, despite repeated military campaigns aimed at destroying it.

No group immediately claimed responsibility for the attack.