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.
A United Nations-backed war crimes tribunal in Cambodia has handed down long-awaited guilty verdicts and life sentences for two aging leaders of the Khmer Rouge regime. VOA's Say Mony reports from Phnom Penh in this story narrated by Colin Lovett.
More than 1.7 million people died under the regime, from overwork, starvation or execution, in one of the worst atrocities of the 20th century.
In the countryside, especially in former Khmer Rouge areas, the verdict was met with mixed reactions.
The first phase of a two-phase trial began for Nuon Chea and Khieu Samphan in late 2011, in a trial that included more than 1,000 witnesses and 7,000 documents.
Nuon Chea, Khieu Samphan convicted of crimes against humanity three-and-a-half decades after communist group's bloody rule left nearly quarter of Cambodian population dead.
The tribunal sentenced Nuon Chea and Khieu Samphan, who are in poor health, to life in prison
Nuon Chea, 87, and Khieu Samphan, 82, are facing charges for their roles as leaders within the movement