The Tuol Sleng Genocide Museum is a museum for the Cambodian genocide. Located in Phnom Penh, the site is a former secondary school that had been turned into an internment facility known as Security Prison 21 (S-21) by the Khmer Rouge regime from 1975 until its fall in 1979. From 1976 to 1979, an estimated 20,000 people were imprisoned at Tuol Sleng; it was one of between 150 and 196 torture and execution centers established by the Khmer Rouge and their secret police.