The United Nations-sponsored tribunal is the second trial for former head of state Khieu Samphan and chief ideologue Nuon Chea, both in their 80s. Each was tried last year of crimes against humanity, involving forced evacuations and the mass executions of soldiers, and will be sentenced Aug. 7.
The current trial will concern mass killings of thousands of Cambodia's Vietnamese and Cham Muslim minorities during the 1975-1979 rule of the Khmer Rouge. An estimated two million people were killed during the attempt to create a socialist utopia in Cambodia.
Leader Pol Pot died in 1998.
Khmer Rouge prison chief Kaing Guek Eav, who operated the Tuol Sleng torture and execution center in Phnom Penh, is serving a sentence of life imprisonment after the tribunal convicted him for his role in killing more than 14,000 people.