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.
The ceremony will include prayers and will allow participants to lay wreaths at the 'killing fields', Cheoung Ek graves.
Investigating judges have been criticized for hastily concluded Case 003, without field investigations into some sites.
The defendants in this case, the court’s second, deny all charges.
The Khmer Rouge Tribunal is caught up in a spat between its investigating judges and international prosecutor over case 003.