The Khmer Rouge killed at least 1.7 million people, and singled out ethnic minorities – such as the Cham Muslims and Vietnamese – for particularly brutal repression.

Nonn defined the regime’s goal as a “social revolution” that they hoped would result in an “atheistic and homogenous population of local peasants.”

The convictions were the international tribunal’s first for genocide, a crime that is notoriously difficult to prosecute in international courts. Chea, also known as Brother No 2, was Pol Pot’s second-in-command, while Samphan was the regime’s head of state.

Khmer Rouge trial nears the end and it may be the last of its kind as few senior leaders remain alive

Both were already serving life sentences for their 2014 convictions for crimes against humanity, among other offences.

The court found both Chea and Samphan guilty of genocide against ethnic Vietnamese, but only Chea guilty of genocide against the Cham Muslims.

The judge explained that although Chea may not have actively ordered genocide against Cham Muslims, he likely knew of and could have stopped the killings because of his senior status in the Khmer Rouge and “superior responsibility”.

The court, on the other hand, found that Samphan’s role as head of state was largely nominal and, as a result, he was not in a senior enough position to have stopped the Cham genocide.

The UN defines “genocide” as action pursued in the eradication of “a national, ethnical, racial or religious group”. This means that Khmer Rouge crimes against other Khmer did not qualify.

The court, officially known as the Extraordinary Chambers in the Courts of Cambodia (ECCC), ruled that the Khmer Rouge targeted the Cham and Vietnamese “not as individuals, but based on membership in a group”, constituting genocide.

Established in 2006 as a joint effort of the Cambodian government and the UN, the ECCC has convicted just three men at a cost of about US$300 million. The court has often been criticised for its limited authority to prosecute only “those most responsible”, its slow pace and for alleged political inference.

Sopheap Chak, the executive director of the Cambodian Center for Human Rights, said in an interview that the tribunal’s flaws are typical of Cambodian courts.

“Despite its flaws, the ECCC could still serve as an example for other courts in Cambodia,” Chak said. She praised the court’s protection of victims, recognition of gender-based violence and sexual crimes, and relative transparency, which she said was rare in the country’s judicial system.

Three other Khmer Rouge leaders – Ao An, Meas Muth, and Yim Tith – still face genocide charges before the ECCC. Even so, political pressure and international fatigue may render Friday’s judgment the tribunal’s swan song.

Prime Minister Hun Sen, himself a former Khmer Rouge commander, has objected to these additional cases, known as Cases 003 and 004, claiming at one point that bringing them to court would cause “a civil war” that would kill “200,000 to 300,000 people.”

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But others disagree.

“I need the ECCC to continue with further trials against lower-level Khmer Rouge,” said Arun Sothea, 46, a survivor of Khmer Rouge brutality that left him an orphan. “I don’t care about any opposition of political leaders … What I care about is to find true justice for my whole family and other victims.”

Financial strain has also imperilled the tribunal, with international donors becoming increasingly frustrated by its pace and perceived dysfunction. Most of the funding has come from Japan, the United States and Australia. China, Cambodia’s largest foreign benefactor, has not contributed.

Asked on Thursday if the tribunal would pursue Cases 003 and 004, Neth Pheaktra, the ECCC’s spokesman, said that “the work of the court will continue.”

Others within the tribunal are more confident still. One senior tribunal official said that “cases 003 and 004 will at the very least find their way to the trial chamber.”