PHNOM PENH, Cambodia, Oct. 25 — He had a job to do, and he did it supremely well, under threat of death, within earshot of screams of torture: methodically photographing Khmer Rouge prisoners and producing a haunting collection of mug shots that has become the visual symbol of Cambodia’s mass killings.

“I’m just a photographer; I don’t know anything,” he said he told the newly arrived prisoners as he removed their blindfolds and adjusted the angles of their heads. But he knew, as they did not, that every one of them would be killed.

“I had my job, and I had to take care of my job,” he said in a recent interview. “Each of us had our own responsibilities. I wasn’t allowed to speak with prisoners.”

That was three decades ago, when the photographer, Nhem En, now 47, was on the staff of Tuol Sleng prison, the most notorious torture house of the Khmer Rouge regime, which caused the deaths of 1.7 million people from 1975 to 1979.