In 1975 the Khmer Rouge turned a high school building into a prison to house its enemies. Over four years 17,000 people came through Tuol Sleng's doors. All but a few were executed.
The Khmer Rouge kept detailed records of the prisoners, all of whom were photographed when they arrived at the jail. As the pictures show, some of the prisoners were very young.
Prisoners were kept in cramped cells. They were poorly fed and frequently beaten. Some died from the treatment they received.
More senior prisoners were kept in special rooms where they could be tortured. Some were tied to bedsteads and beaten. Others were suspended from the ceiling or repeatedly submerged in water.
When the jailers had forced a confession, prisoners were taken to fields at Choeung Ek and killed. So far, the remains of nearly 9,000 people have been found there.
A memorial to the victims has been built at Choeung Ek. It is filled with the skulls of some of those who died.