One of last century’s most brutal killing centers was S-21, a prison complex in Phnom Penh where as many as 30,000 innocents were murdered during Cambodia’s genocide led by the Khmer Rouge—an ultra-radical Maoist sect. Between 1975 and 1979, the party murdered close to two million people. Like many of the murderous regimes before, Khmer Rouge officials chose to catalog their victims and document their crimes; in the process of killing, Khmer Rouge officials photographed every incoming prisoner. After the regime fell, 6,000 of these photographs of S-21 victims were recovered. Nothing about any of these prisoners is known, other than their ultimate fate.