The reflection of a display case containing mugshots of prisoners superimposed on opposite-facing case at the Tuol Sleng prison in Phnom Penh, Cambodia. Codenamed S-21, Tuol Sleng was the most important of the Khmer Rouge's nearly 200 prisons. An estimated 17,000 people passed through the prison under the regime from 1975 to 1979. Most were brutally interrogated for months, made to confess to a list of often imaginary political crimes, and then murdered in the infamous Killing Fields. March 1, 2012.