The Santebal was in charge of internal security and running prison camps like Tuol Sleng (S-21) where thousands of people were imprisoned, interrogated, tortured and executed.

As early as 1971, the Khmer Rouge or the Communist Party of Kampuchea established the Special Zone outside of Phnom Penh under the direction of Vorn Vet and Son Sen. Sen, later the Deputy Minister for Defense of Democratic Kampuchea, was also in charge of the Santebal, and in that capacity he appointed Comrade Duch to run its security apparatus.

[1] When the Khmer Rouge took power in 1975, Duch moved his headquarters to Phnom Penh and reported directly to Sen. At that time, a small chapel in the capital was used to incarcerate the regime's prisoners, who totaled fewer than two hundred.

In May 1976, Duch moved his headquarters to its final location, a former high school known as Tuol Sleng, which could hold up to 1,500 prisoners.

It was at Tuol Sleng that the major purges of the Khmer Rouge cadres took place and thousands of prisoners were tortured and killed.