Some former detainees of the camps have described being forced to assist guards or face punishment. Auelkhan said she was powerless to resist or intervene.

Asked if there was a system of organised rape, she said: “Yes, rape.”

“They forced me to go into that room,” she said. “They forced me to take off those women’s clothes and to restrain their hands and leave the room.”

Some of the women who were taken away from the cells at night were never returned, Ziawudun said. Those who were brought back were threatened against telling others in the cell what had happened to them.

“You can’t tell anyone what happened, you can only lie down quietly,” she said. “It is designed to destroy everyone’s spirit.”

Mr Zenz told the BBC that the testimony gathered for this story was “some of the most horrendous evidence I have seen since the atrocity began”.