The former chief of the notorious S-21 prison yesterday told of how he swapped painkillers for poison and thwarted an order from his alleged superior – Khmer Rouge tribunal defendant Nuon Chea – in an apparent bid to spare the lives of four inmates.

Kaing Guek Eav, more commonly known by the nomme de guerre “Duch”, returned to the courtroom to answer questions about the Tuol Sleng prison, where more than 12,000 people were imprisoned and later executed.

Duch told the courtroom experimental surgeries performed on live prisoners ceased after 1975; however, he said his superior, whom he identified as Case 002 defendant and former Brother Number Two Nuon Chea, had once handed him four pills to “test” on prisoners.

The pills were discovered in the home of an “enemy” interrogated at S-21, who confessed under torture to a plot to kill or poison Khmer Rouge cadres.

Thousands of questionable S-21 “confessions” were obtained under torture, and detailed prisoners’ alleged connections to international spy agencies like the CIA and KGB.

Duch said he threw out the tablets and ground up paracetamol, then funnelled it into the empty capsules and gave them to inmates. “In fact I changed the powder so it was not poisonous,” he said.

It was one of the small redemptive acts Duch described amid the vast atrocities committed at his prison. Another was when he tried, but ultimately failed, to save three children of a slain prisoner at M13, a prison he oversaw before moving to S-21.

“I could not keep them alive . . . I reported to brother Son Sen and I told him that raising the children at the prison was not successful because we did not have the emotional attachment,” he said.

In response, Duch’s superior, Son Sen, ordered an “absolute” stance, where wives and children were “smashed”.

The prosecution confronted Duch with lists of heavily pregnant women who were sent from the rice fields at Prey Sar to the killing fields of Cheoung Ek, as well as a series of faded photographs depicting an entire family who were arrested and detained at S-21.

The children of that family were all executed within a week of their arrival – although one of the girls committed suicide on Christmas Day after the death of her siblings. Their father was executed nine months later.

When questioned if Son Sen, and later Nuon Chea, condoned or condemned the killing of children, Duch responded: “There was never an instruction not to kill children, not to kill pregnant women.”