PHNOM PENH, Cambodia — Ieng Sary, who cofounded the communist Khmer Rouge regime responsible for the deaths of an estimated 1.7 million Cambodians in the 1970s and who decades later became one of its few leaders to be put on trial, died Thursday at 87 before his case could be finished.
Mr. Ieng Sary was the brother-in-law of late Khmer Rouge leader Pol Pot. His death dashed hopes he would be punished for alleged crimes against humanity during the darkest chapter in his country’s history.
Chea Leang, a co-prosecutor at the Cambodian-international tribunal that had been trying Mr. Ieng Sary, said he died of cardiac failure. The trial began in late 2011 with four defendants and now has two.
Mr. Ieng Sary suffered from high blood pressure and heart problems. His body was taken Thursday to Malai in western Cambodia, a former Khmer Rouge stronghold where his family lives, for his funeral.
There are fears that the two remaining former Khmer Rouge leaders still on trial, both in their 80s, could also die before justice is served. Ieng Sary’s wife, former Social Affairs Minister Ieng Thirith, also was charged but was ruled unfit to stand trial last year because she suffers from dementia, probably Alzheimer’s disease.
‘‘We are disappointed that we could not complete the proceeding against Ieng Sary,’’ tribunal spokesman Lars Olsen said, adding that the case against chief Khmer Rouge ideologist Nuon Chea and former head of state Khieu Samphan would not be affected.
Mr. Ieng Sary founded the Khmer Rouge with Pol Pot.
The regime, which ruled Cambodia from 1975 to 1979, claimed it was building a pure socialist society by evicting people from cities to work in labor camps in the countryside. Its radical policies led to the deaths of an estimated 1.7 million people from starvation, disease, overwork and execution.
‘‘He was a critical part of the senior leadership and his death undoubtedly will have an impact on the case,’’ said Elizabeth Becker, a former New York Times journalist and author of ‘‘When the War Was Over,’’ a history of modern Cambodia and the Khmer Rouge. ‘‘This trial was held 30 years too late.’’
Mr. Ieng Sary was foreign minister in the regime, and as its top diplomat became a much more recognizable figure internationally than his secretive colleagues.
The Khmer Rouge came to power through a civil war that toppled a US-backed government. Mr. Ieng Sary then helped persuade hundreds of Cambodian intellectuals to return home from overseas to help the new regime.
The returnees were arrested and put in ‘‘reeducation camps,’’ and most were later executed, said Youk Chhang, director of the Documentation Center of Cambodia, an independent group gathering evidence of Khmer Rouge crimes.
As a member of the Khmer Rouge’s central and standing committee, Mr. Ieng Sary ‘‘repeatedly and publicly encouraged, and also facilitated, arrests and executions within his Foreign Ministry and throughout Cambodia,’’ Steve Heder said in his coauthored book ‘‘Seven Candidates for Prosecution: Accountability for the Crimes of the Khmer Rouge.’’
Known by his revolutionary alias, ‘‘Comrade Van,’’ Mr. Ieng Sary was a recipient of many internal Khmer Rouge documents detailing torture and mass execution of suspected enemies, according to the Documentation Center of Cambodia.
‘‘We are continuing to wipe out remaining [internal enemies] gradually, no matter if they are opposed to our revolution overtly or covertly,’’ read a cable sent to Mr. Ieng Sary in 1978. It was reprinted in the center’s magazine in 2000, apparently proving he had full knowledge of bloody purges.
In 1996, years after the overthrown Khmer Rouge retreated to the jungle, Mr. Ieng Sary became the first member of its inner circle to surrender, bringing thousands of foot soldiers with him and hastening the movement’s final disintegration.
‘‘He liked good food, power, money, nice living, and the idea, mistaken, that communism was the wave of the future,’’ said David Chandler, of Australia’s Monash University. ‘‘He was a wily and able foreign minister. He quit when the quitting was good.’’
Mr. Ieng Sary’s surrender secured him a limited amnesty, temporary credibility as a peacemaker, and years of comfortable living in Cambodia, but that vanished as the UN-backed tribunal built its case.
Mr. Ieng Sary was arrested in 2007, and the trial against him started in late 2011. He faced charges that included crimes against humanity, war crimes, and genocide.
He denied any hand in the atrocities, saying Pol Pot ‘‘was the sole and supreme architect of the party’s line, strategy, and tactics.’’
He claimed that he was a secondary figure excluded from Pol Pot’s secret security committee, which decided policy and who would be executed.
‘‘Do I have remorse? No,’’ he said in 1996. ‘‘I have no regrets because this was not my responsibility.’’
Only one former Khmer Rouge official has been tried and convicted: former prison chief Kaing Guek Eav, who was sentenced to life in prison.
Prime Minister Hun Sen has openly opposed additional indictments of former Khmer Rouge figures, some of whom have become his political allies.
Pol Pot died in 1998.