S-21 served as the Khmer Rouge regime‘s highest level security center. The regime used the prison to interrogate, torture and execute those considered to be its traitors. Today also is known as “Kuk Tuol Sleng” or “Tuol Sleng prison“, the former detention center has been transformed into the Tuol Sleng Genocide Museum. Historians try to shed a light on the history of this place. But still, many details about the prison and its operation are unclear, even the actual size of the prison.