S-21 prison photographer Nhem Ein holds an identification photograph of himself dressed in Khmer Rouge uniform, made in 1976, when he was only 16. Nhem Ein was sent for specialized photographic training in Shanghai when he was 15; when he returned to Cambodia in 1976 he immediately began taking photographs of prisoners imprisoned by Pol Pot. Each day Ein had to photograph, develop, and print photographs of each incoming prisoner, which Ein or a colleague delivered to Son Sen, a French-trained revolutionary who was number three in the Khmer Rouge leadership. Ein said he photographed more than 18,000 prisoners, though he recalled a cumulative prison population of nearly 30,000. Nhem Ein remained with the Khmer Rouge until 1996, when American researcher Doug Niven identified Ein after reading a local newspaper story in Phnom Penh. Photo: © Doug Niven