Prior to 1975, Toul Sleng was a high school. When the Khmer Rouge came to power it was converted into the S-21 prison and interrogation facility. Inmates were systematically tortured to extract confessions, after which they were executed at the killing fields of Choeung Ek. S-21 processed over 17,000 people, less than a dozen of whom survived. The building now serves as a museum, a memorial and a testament to the madness of the Khmer Rouge regime.