PHNOM PENH, Cambodia — Three and a half decades after the genocidal rule of Cambodia’s Khmer Rouge ended, a U.N.-backed war crimes tribunal is due to deliver its first verdicts Thursday in a historic case against the only two leaders of the regime left to stand trial.

Khieu Samphan, the regime’s 83-year-old former head of state, and Nuon Chea, its 88-year-old chief ideologue, face sentences ranging from five years to life for their role in the 1970s terror. Both men, in dire health, have denied any wrongdoing.

The case, covering the forced exodus of millions of people from Cambodia’s towns and cities and a mass killing, is just part of the Cambodian story. Nearly a quarter of the population died under their rule, through a combination starvation, medical neglect, overwork and execution when the group held power in 1975-79.

Many have criticized the slow justice, and its cost. The tribunal, formally known as the Extraordinary Chambers in the Courts of Cambodia and comprising of Cambodian and international jurists, began operations in 2006. It has since spent more than $200 million, yet it has only convicted one defendant – prison director Kaing Guek Eav, who was sentenced to life imprisonment in 2011.

The current trial began in November 2011 and started out with four Khmer Rouge leaders. Former Foreign Minister Ieng Sary died in 2013, while his wife, Social Affairs Minister Ieng Thirith, was deemed unfit to stand trial due to dementia in 2012. The group’s top leader, Pol Pot, died in 1998.

Khieu Samphan and Nuon Chea have both been charged with crimes against humanity, genocide, religious persecution, homicide and torture. Any sentence is likely to be a death sentence since both men are in frail health and have required occasional hospitalization during the trial.

Khieu Samphan has acknowledged that mass killings took place. But testifying before the court in 2011, he claimed he was just a figurehead who had no real authority. He denied ordering any executions himself, calling the allegations a “fairy tale.” Instead, he blamed Pol Pot for its extreme policies.