Halfway through a short speech he was giving at the German Bundestag, Zhang Yabo began to choke back tears.

Zhang had served for nine years as a police officer in Xinjiang, an enforcer in China’s campaign to surveil, detain and indoctrinate that region’s Uyghur population. At the campaign’s height, an estimated 1 million Uyghurs and other Muslim minorities were held in internment camps.

Tens of thousands of police officers carried out China’s state oppression in Xinjiang, but Zhang is a rare voice speaking out about it. Standing before German lawmakers and human rights activists in June, Zhang described Xinjiang as a giant prison enclosed by an “ironclad wall.”