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 center 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 died during torture were buried in mass graves in the prison grounds. S-21 has been turned into the Tuol Sleng Museum, which serves as a testament to the crimes of the Khmer Rough.
The museum's entrance is on the western side of 113 St just north of 350 St, and it is open daily from 7 to 11.30 am and from 2 to 5.30 pm; entry is US$3.Like the Nazis, the Khmer Rough was meticulous in keeping records of their barbarism. Each prisoner who passed through S.21 was photographed, sometimes before and after being tortured. The museum displays include room after room in which such photographs of men, women and children cover the walls from floor to ceiling; virtually all the people pictured were later killed.

You can tell in what year a picture was taken by the style of number board that appears on the prisoner's chest. Several foreigners from Australia, France and the USA were held here before being murdered. Their documents are on display. As the Khmer 'revolution' reached ever-greater heights of insanity, it began devouring its own children. Generations of tortures and executioners and were in turn killed by those who took their places. During the first part of 1977, S-21 claimed an average of 100 victims a day. When the Vietnamese army liberated Phnom Penh in early 1979, they found only seven prisoners alive at S-21. Fourteen others had been tortured to death as Vietnamese forces were closing in on the city. Photographs of their decomposing corpses were found. Their graves are nearby in the courtyard.
Altogether, a visit to Tuol Sleng is a profoundly depressing experience. There is something about the sheer ordinariness of the place that make it even more horrific; the suburban setting, the plain school buildings, the grassy playing area where several children kick around a ball, ousted beds, instruments of torture and wall after wall of harrowing black-and-white portraits conjure up images of humanity at its worst. Tuol Sleng is not for the squeamish.