From 1976 to 1979, an estimated 20,000 people were imprisoned at Tuol Sleng (the real number is unknown). At any one time, the prison held between 1,000–1,500 prisoners. They were repeatedly tortured and coerced into naming family members and close associates, who were in turn arrested, tortured and killed. In the early months of S-21's existence, most of the victims were from the previous Lon Nol regime and included soldiers, government officials, as well as academics, doctors, teachers, students, factory workers, monks, engineers, etc. Later, the party leadership's paranoia turned on its own ranks and purges throughout the country saw thousands of party activists and their families brought to Tuol Sleng and murdered. Those arrested included some of the highest ranking politicians such as Khoy Thoun, Vorn Vet and Hu Nim. Although the official reason for their arrest was "espionage", these men may have been viewed by Khmer Rouge leader Pol Pot as potential leaders of a coup against him. Prisoners' families were sometimes brought "en masse" to be interrogated and later executed at the Choeung Ek extermination center.