Some of the many photos on display at Tuol Sleng prison in Phnom Penh. Once a high school, Tuol Sleng was used by the Khmer Rouge in the 1970s to interrogate political opponents before sending them to their deaths in the killing fields of Choeung Ek. Of the 17,000 inmates who entered Tuol Sleng, less than 10 people are known to have survived.