The Khmer Rouge took control the country on 17, April 1975 and people were forced to evacuate the city to countryside. In the same year, 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, the prison was turned into a genocide museum.
Source: Chandler, David. The Tragedy of Cambodian History: Politics, War, and Revolution