In his award-winning documentary, “Enemies of the People,” Thet Sambath faces Nuon Chea and asks why some people were killed.
During the Khmer Rouge rule, fear descended on villages as people were accused of being counter-revolutionaries and began disappearing.
In one of the last strongholds of the Khmer Rouge, some Cambodians still consider Pol Pot a powerful figure to be worshipped.
All four have said they are innocent of the charges against them, including war crimes, crimes against humanity and genocide.
In 1996 Khmer Rouge foreign minister Ieng Sary defected to the Cambodian government after receiving a royal pardon and amnesty.
Stephen Rapp said their trial has been a high priority for US Secretary of State Hillary Clinton.
Ieng Sary is charged with war crimes, crimes against humanity, genocide and other crimes related to the Khmer Rouge leadership.
During Cambodia’s bloody Khmer Rouge era in the 1970s, religion was outlawed.
Thet Sambath spent more than a decade in extensive interviews with Nuon Chea before his arrest and detention at the UN-backed tribunal.
Monday’s hearing—a preliminary, mostly procedural affair—marks the beginning of a trial that Cambodians have awaited for decades.
The four elderly defendants, aged 79 to 85, all deny the charges against them. Their trial could last a few years.
The national and international lawyers have said they are skeptical about the interpretation of the laws governing the tribunal.