A notorious Khmer Rouge jailer has begun to testify against the top three surviving members of the party that allegedly imposed a reign of terror on Cambodia during the late 1970s.

Former prison warden Kaing Guek Eav, better known as Duch , is the only person so far to have been convicted by the international tribunal trying Khmer Rouge war crimes.

During questioning Monday, Duch described how one of his immediate superiors instructed him how to interrogate prisoners: how to torture them using electrical wiring, water-boarding techniques and suffocation.

Authorities say Duch provided much of the evidence against nominal Khmer Rouge head of state Khieu Samphan, reputed chief ideologue Nuon Chea and Khmer Rouge government minister Ieng Sary, who are facing an array of charges linked to their rule.

All three were expected to be in the courtroom in Phnom Penh to hear the testimony of Duch, who was recognized for his cooperation and candid responses to questions at his own trial.

Duch was sentenced to life in prison last month for overseeing the notorious Khmer Rouge Tuol Sleng torture prison, where as many as 14,000 Cambodians died. 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 regime ruled Cambodia from 1975 to 1979, and is blamed for the deaths of some 1.7 million people by overwork, starvation, torture or execution.

Also Monday, a second international judge resigned from the tribunal, complaining of improper interference in his work.

Laurent Kaspar-Ansermet said he could not continue because of repeated challenges to his authority by his Cambodian counterpart on the tribunal, You Bunleng.

Just months earlier, German investigating judge Siegfried Blunt resigned complaining of political interference.