In 1975,Tuol Svay Prey High School was taken over by Pol Pot's security force and turned into a prison known as Security Prison 21 (S-21) It soon became the largest such centre of detention and torture in the country. Over 17,000 people held at S-21 were taken to the extermination camp at Choeung Ek to be executed; detainees who die during torture were buried in mass graves in the prison grounds.
