Inscription: (Plaque above a room's door in Building A) BUILDING "A". It contains three stories, divided into 20 cells: the first one has 10 cells, used for jailing, interrogating and torturing the prisoners who had been the high officials. The second and third ones have 5 big cells each, used for the same purpose as on the first story. Nowadays there are a lot of evidences remaining in all the cells, which prove the atrocities of Pol Pot clique.
In 1975 the Khmer Rouge turned Tuol Svay Prey High School into a prison and interrogation center known as Security Prison 21. It became the largest detention center in the country, and out of almost 20,000 prisoners detained there only seven survived. When the Vietnamese liberated Phnom Penh in 1979, they turned the prison into a genocide museum.