The testimony of 63-year-old It Sen, an ethnic Cham who took to the witness stand on Monday, September 8, made for grim listening. For centuries, the Cham people have comprised a distinct ethnic group, with their own language, customs, dress and – not least - religion: in a predominantly Buddhist nation, they follow Islam.

But, Sen told the court, all that changed when Pol Pot's Khmer Rouge took control of Ampil Village in eastern Cambodia in 1973, halfway through the civil war that saw the brutal movement seize power two years later. The Khmer Rouge immediately instituted a range of oppressive measures in Sen's village, barring residents from practising their religion and customs, and even from speaking their own language.

"Only the Khmer language was allowed to [be spoken] at that time," he said. "We could speak Cham, but, you know, in a secret way – not loudly. If they happened to hear us speaking the Cham language we would be taken away and killed."

The Khmer Rouge desecrated or destroyed mosques, burned copies of the Koran and other sacred Cham texts, and executed Cham leaders. They also forced the people to dress in Khmer Rouge-black, to cut their hair short, and to eat pork and raise pigs. Execution was the standard punishment for breaching the new rules. "If we were even to make the smallest mistake we would be arrested and killed," Sen said.

After a Cham uprising in 1975 on the nearby Mekong island of Koh Phal, the Khmer Rouge exacted a terrible revenge, murdering the men of the island and deporting tens of thousands of the province's Cham villagers to sites across the country. Among those sent away were Sen and his young family.

Sen, who is a farmer, is the first witness whose testimony the court is hearing on the charge of genocide, one of a multitude of alleged crimes the two surviving leaders of the Khmer Rouge have been charged with: Nuon Chea, who was Pol Pot's deputy; and Khieu Samphan, the regime's head of state.

The tribunal last year found the two defendants guilty of war crimes and crimes against humanity after the close of the first part of their trial. Both men have appealed that verdict.

Genocide in the 'Cambodian context'

The Khmer Rouge's rule of Cambodia was one of the bloodiest of the 20th century. By the time Pol Pot's regime was overthrown in January 1979, according to Farina So, Director for the Centre of Gender and Ethnic Studies at the Documentation Center of Cambodia (DC-Cam), around 36 percent of the estimated pre-war Cham population of 300,000 were dead.

Figures for the pre-war Cham population vary widely, but that death rate, if accurate, would be significantly higher than that for the general population: the Khmer Rouge are thought to have caused the deaths of 2 million people - one in four Cambodians - between 1975-79. Around half were executed; the remainder succumbed to overwork, starvation and disease.

Importantly – and no doubt surprisingly to many readers – the charge of genocide at Cambodia's UN-backed war crimes court does not refer to the mass killings of hundreds of thousands of ordinary Cambodians, the ethnic Khmer who comprised the majority of the population and the largest number of victims.

Instead, the charge of genocide relates solely to the Khmer Rouge's treatment of two minority groups: Cham Muslims and ethnic Vietnamese.

Lars Olsen, the tribunal's legal communications officer, said that is because the definition of genocide in law "is different from what many people would regard as genocide." Simply put, the 1948 Genocide Convention defines genocide as having the intention to eliminate, in part or in full, a group of people based on their race, religion, ethnicity or nationality.

"Clearly in this context of Cambodia where the majority of those killed during the Khmer Rouge regime were killed by Cambodians with the same ethnicity, the traditional legal definition of genocide would not include these kinds of acts," Olsen said.

Prosecution's contention

In other words, because the accepted definition of genocide requires that you act against others, the Khmer Rouge's mass killing of ethnic Khmers precludes that from being considered genocide. On the other hand their victimization of Cham Muslims and ethnic Vietnamese, the prosecution contends, does constitute genocide.

The prosecution says 90 percent of the 200,000-strong ethnic Vietnamese population in Cambodia were forcibly deported during the Khmer Rouge's rule, and that the remaining 20,000 were largely exterminated by 1979 in a process that "involved mass killings of Vietnamese civilians who were sought out solely on the basis of their ethnicity."

The prosecution's docket notes that the Khmer Rouge's actions "succeeded in physically destroying a significant portion of the Cham population, solely because of their ethnic and religious background."

"This effort included removing Cham women and children from the group and placing them in Khmer communities, forcing Cham to marry outside their ethnic group, and banning all cultural aspects traditionally identified with that group," it states, adding that this persecution also included forced removals and executing Cham leaders.

"This campaign culminated in 1977 and 1978 with organized mass executions of entire Cham communities conducted by [Khmer Rouge] cadres in multiple [areas of Cambodia]," the document concludes.

It should be said, however, that some researchers doubt the actions against the Cham amount to genocide, noting that while they did suffer terribly under the Khmer Rouge their experience was not significantly worse than that of other Cambodians.

'The village was full of soldiers'

Back in court, Sen told the bench that after the 1975 Koh Phal uprising he was sent north with his family where they lived for three years. Then, in 1978, the village chief ordered them to return home.

But soon after arriving at Ampil Village, the authorities there told him and other Cham families they must leave for Treah Village. Sen, his wife and their three-year-old son joined 20 other Cham families on a one-day trek to Treah by ox cart.

As the families neared Treah, they heard disturbing stories about Cham being blindfolded at the village and taken to the edge of the Mekong River. They kept going, though, and reached Treah after sunset. The village, he recalled, was "full of soldiers," who split up the families at gunpoint. Sen and several dozen other men were taken to a nearby house where they were beaten and then tied up.

Early the next morning he watched, horrified, as groups of Cham men, several dozen at a time, were taken from adjacent houses blindfolded, their wrists tied, to the riverbank where a rope was threaded through the men's arms. Then, the Khmer Rouge tied one end of the rope to the stern of a motorboat, removed the men's blindfolds, and the boat roared off for the middle of the Mekong.

The men fell to the ground and were dragged into the river where they drowned. Then a cadre untied the rope on the stern, and the boat returned to drown a new group of blindfolded Cham men. The killings went on all day. "I was crying, together with the other Cham people [in the house]," he told the court.

The men, terrified, knew their time would soon come; with one group already taken from house, and with night falling, Sen wriggled free and escaped the house unnoticed by slipping between wooden boards.

As he fled to a stand of nearby bushes, Sen passed a pile of dead prisoners' clothes two metres wide and "as high as my waist." He hid in the bushes, not far from burial pits littered with iron bars for killing people, before getting away by swimming the river. Sen was one of the few from his village to survive the regime.

"I never saw my wife again," the softly spoken witness told the court. "And I conclude that my wife and my child who was with her died. However I do not know how they died. It is possible that they were taken away and killed [in the burial pits] or that they were drowned."

The trial of Nuon Chea and Khieu Samphan is expected to continue into 2016.