The photos were donated anonymously this month to the Documentation Center of Cambodia. They stunned archivists with rich details of prisoners held at the Tuol Sleng prison during the rule of the extremist group. Documentation center chief Chhang Youk tells VOA's Khmer Service the back of each passport-size photo contains information on each prisoner, with much of it written by the prisoners themselves. Tuol Sleng, known to the Khmer Rouge as S, was a place of particular horror. Of its estimated 17, prisoners, only seven survived, making it the regime's deadliest venue.