The Tuol Sleng Genocide Museum is a museum in Phnom Penh, the capital of Cambodia. The site is a former high school which was used as the notorious Security Prison 21 (S-21) by the Khmer Rouge regime from its rise to power in 1975 to its fall in 1979. From 1975 to 1979, an estimated 17,000 people were imprisoned, tortured and killed at Tuol Sleng.