Nuon Chea is on trial alongside Khieu Samphan, another regime leader, for atrocity crimes.
Nearly half a million Cambodians fled the regime to live in the United States, leaving behind family members who were killed.
Scot Marciel, the top diplomat for Asia and Pacific at the US State Department, said the tribunal can serve as an example to Cambodians and the world.
Observers say they are hopeful the case will reveal more truths about the movement and provide some sense of justice for victims.
A researcher says interviews with many everyday Cambodians shows a desire to hear confessions from two former leaders.
Im Chaem, 68, is accused of atrocity crimes for her role in purges of Khmer Rogue cadre and for running a detention center where tens of thousands of people died.
The fact that only three leaders have faced trial since 2006 means the court is failing to bring justice to the victims of the brutal regime.
Im Chaem has said in the past she does not consider herself guilty of atrocity crimes, and she recently told local media she will not go to the court if summoned.
Im Chaem, 68, is among a small group of suspects that could be indicted in two more cases at the tribunal.
A United Nations-backed war crimes tribunal in Cambodia has handed down long awaited guilty verdicts against two aging leaders of the Khmer Rouge regime.
While the verdict signaled a completion to this phase of the trial, critics say the court could have done a lot more for Cambodians.