S-21 or Tuol Sleng Prison is one of many prisons where people was interrogated and tortured during the "Khmer Rouge" years. Those who survived S-21 went directly to the "killing fields" of Choeung Ek for excecution.
17000 men, women and children were excecuted at Choeung Ek. CE was just ONE of Pol Pots more than 300 "killing fields" in Cambodia.