The Tuol Sleng Genocide Museum is a museum in Phnom Penh The site is a former high school which was used as the notorious Security Prison 21. Upon arrival at the prison, prisoners were photographed and required to give detailed autobiographies, beginning with their childhood and ending with their arrest. After that, they were forced to strip to their underwear, and their possessions were confiscated. The prisoners were then taken to their cells. Those taken to the smaller cells were shackled to the walls or the concrete floor.