Kaing Guek Eav, the chairman of the infamous Tuol Sleng prison and more commonly known as Comrade Duch, is set testify at the Khmer Rouge tribunal next week.

Duch was the first Khmer Rouge leader convicted of crimes against humanity by the court over his role at the Phnom Penh prison, also known as S-21, where more than 12,000 people were detained, tortured and killed.

He is scheduled to testify over nine days, starting on Thursday next week, on the security centre and internal purges during the regime in the current Case 002/02 against former head of state Khieu Samphan and “Brother Number Two” Nuon Chea.

Duch first appeared as a witness in Case 002/01 one month after he was sentenced to life in prison in February 2012.

Trial observer Long Panhavuth said he doubted Duch’s life sentence would taint his testimony.

“I believe Duch is not a malicious witness, but I cannot see the benefit or that he will add something new,” he said.

But Nuon Chea’s defence lawyer, Victor Koppe, said many questions remained unanswered.

“There are still a lot of questions to be asked, especially about the whole chain of command issue – he is not very credible when it comes to that,” Koppe said, referring to past testimony that implicated Nuon Chea as Duch’s superior from August 1977.