An ex-Khmer Rouge prison warden has identified the group's one-time chief ideologue, Nuon Chea, as his former superior at the notorious Tuol Sleng prison, where thousands of the regime's perceived enemies were tortured and executed.

Kaing Guek Eav, better known as Duch , delivered the testimony Tuesday at the long-running trial of Nuon Chea, nominal Khmer Rouge head of state Khieu Samphan and Khmer Rouge government minister Ieng Sary. The three are the most senior surviving leaders of the group that brutalized Cambodia in the late 1970s.

Duch, who was convicted earlier by the international tribunal, said on his second day of testimony that Nuon Chea had been his superior at the prison, also known as S-21, for at least part of the time he was there.

He also acknowledged that he knew some of the intellectuals arrested and sent to S-21 during its early days were not guilty of anything.

On his opening day of testimony Monday, Duch described how he had been taught by the Khmer Rouge how to interrogate prisoners, including torture using electrical wiring, waterboarding techniques and suffocation.

On Tuesday, he elaborated on his understanding of Khmer Rouge ideology, which was to “isolate” the group's enemies “and have them smashed.”

Duch, who did not try to defend his actions at his own trial, is expected to provide key evidence during his testimony which is scheduled to last all week.

Duch was sentenced to life in prison last month for overseeing Tuol Sleng. In handing down the sentence, the tribunal called the prison “a factory of death,” and described Duch as “a shocking and heinous character” who deserved “the highest penalty available.”

The ultra-Maoist Khmer Rouge ruled Cambodia from 1975 to 1979, and is blamed for the deaths of some 1.7 million people by overwork, starvation, torture or execution.