Duch said he had picked up beating techniques from Cambodian and French police manuals and worked out his own system through trial and error. In passing on his techniques, he said, he often had to instruct young recruits not to get carried away during torture and kill the prisoner. When this happened in Tuol Sleng, the interrogator himself could be sentenced to death.
While running the prison, he was also appointed to head the Santebal, the “special branch” in charge of internal security and prisons.
When a Vietnamese invasion drove the Khmer Rouge from Phnom Penh in January 1979, Duch oversaw the execution of the remaining prisoners. But he did not destroy the records of interrogations, carefully kept accounts that could run to as many as 200 pages. They amounted, in the end, to his life’s work.
After fleeing Phnom Penh, Duch appeared to undergo a dramatic life change, converting to Christianity while living in refugee communities along the border with Thailand. As a religion of forgiveness, Christianity, if his embrace of it was genuine, may have appealed to a sense of guilt. He sometimes carried a Bible into the courtroom during his trial.
Duch was discovered in 1999 by the photographer Nic Dunlop, who later wrote a book about him, “The Lost Executioner” (2005).
Before being arrested shortly afterward, Duch told Mr. Dunlop and the journalist Nate Thayer that he had decided to confess to prove that the prison, known to the regime as S-21, had really existed, rebutting a claim by the Khmer Rouge leader Pol Pot that accounts of it were propaganda fiction.
During his trial, however, Duch seemed to doubt the validity of his work, telling the courtroom that while running the prison he did not believe most confessions that his torturers had extracted and that he then annotated and sent to his superiors.