A mass grave containing the bodies of dozens of people believed to be Yazidis held captive by Islamic State has been found in territory recently captured from the jihadists by the U.S.-backed Syrian Democratic Forces (SDF), an SDF commander said.

The grave was found in territory seized from Islamic State in Baghouz, the final enclave held by the group near the Iraqi border, SDF commander Adnan Afrin told Reuters.

“They were slaughtered,” he said. Most had been decapitated, he added.

Read more: Reuters