The Center for Justice and Accountability is helping organize applications for victims of the regime who fled to the US.
Khieu Samphan acknowledged the Cambodian people had a pressing need to know what had happened.
Khieu Samphan also pleaded with the court to allow the witnesses submitted by his defense team.
A preliminary hearing this week has brought the amnesty deal made in 1996, to the forefront.
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.