Formerly the Tuol Svay Prey High School, the five buildings of the complex were converted in August 1975, four months after the Khmer Rouge won the civil war, into a prison and interrogation centre. The Khmer Rouge renamed the complex “Security Prison 21” (S-21) and construction began to adapt the prison to the inmates: the buildings were enclosed in electrified barbed wire, the classrooms converted into tiny prison and torture chambers, and all windows were covered with iron bars and barbed wire to prevent escapes.
From 1975 to 1979, an estimated 17,000 people were imprisoned at Tuol Sleng. The prisoners were selected from all around the country, and usually were former Khmer Rouge members and soldiers, accused of betraying the party or revolution. Prisoners’ families were often brought en masse to be interrogated and later murdered at the Choeung Ek (the Killing Fields).
Even though the vast majority of the victims were Cambodian, foreigners were also imprisoned, including Vietnamese, Laotians, Indians, Pakistanis, Britons, Americans, New Zealanders and Australians.
In 1979, the prison was uncovered by the invading Vietnamese army. In 1980, the prison was reopened as a historical museum. The museum is open to the public, and receives an average of 500 visitors every day.