A former chief jailer for the notorious Khmer Rouge jailer has testified against the top three surviving members of the group that imposed a reign of terror on Cambodia during the late 1970s.

Kaing Guek Eav, better known as Duch , testified Monday in the Cambodian capital, Phnom Penh.

During questioning, he said he was the reluctant head of a notorious Khmer Rouge prison, called M-13, used to interrogate, torture and kill. He also said one of his immediate supervisors instructed him how to interrogate prisoners, including torture using electrical wiring, waterboarding techniques and suffocation.

Duch later brought the same techniques to the notorious Tuol Sleng torture center in Phnom Penh, known as S-21, where more than 12,000 Cambodians perished. Last month, the U.N.-backed tribunal for crimes during the Khmer Rouge era found Duch responsible for their deaths and sentenced him to life in prison.

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.”

Also Monday, a second international judge resigned from the tribunal, citing alleged staunch opposition from his Cambodian colleague, You Bunleng, to the crucial investigation of new suspects. Swiss co-investigating judge Laurent Kaspar-Ansermet deplored what he called a “dysfunctional situation” within the court.

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

The Khmer Rouge, the ultra-Maoist regime, ruled Cambodia from 1975 to 1979, and is blamed for the deaths of nearly 2 million people by overwork, starvation, torture or execution.