With the Cambodian king, Norodom Sihanouk, back in power by the early 1990s, Mr. Ieng became the first leader of the Khmer regime to defect from the old movement. In what was presented as a gesture of national reconciliation, he secured a pardon by the monarch in 1996 absolving him of genocide.
Over the next many years, Mr. Ieng lived freely and lavishly at homes in Phnom Penh and the Cambodian city of Pailin. He donated generously to Buddhist temples and the ruling Hun Sen party.
The U.N. tribunal formed in 2003, and Mr. Ieng was arrested four years later. Court proceedings began in 2011.
“The regime was highly centralized, and he was one of the top several figures at the center of it,” Ben Kiernan, director of Yale University’s Genocide Studies Program, said of Mr. Ieng.
Khmer Rouge leaders, Kiernan added, “used a claim that the violence was beyond their control and spontaneous and grass roots. There was some of that at first, but the killing escalated the more the regime established control. In the last year of their regime . . . the killing was at its highest level. By that time, there was very little spontaneous activity of any kind.”
Mr. Ieng was born Kim Trang in Chau Thanh, in southern Vietnam, on Oct. 24, 1925, to a Cambodian father and a Chinese mother.
Mr. Ieng’s relationship with Pol Pot began when they were students at a French school in Phnom Penh. They subsequently received government scholarships to study in France, where in the early 1950s they fell in with student radicals and embraced a violent communist ideology.
Mr. Ieng took the revolutionary name of “Comrade Van” and changed his name to Ieng Sary after moving to Cambodia in 1957. At first he worked as a high school history and geography teacher in Phnom Penh, but his involvement in what became the Khmer Rouge soon became the focus of his life. Amid a brutal crackdown on communism by King Sihanouk, Mr. Ieng moved to northeastern Cambodia to live as a revolutionary deep in the jungle for seven years.
By the early 1970s, amid the deepening Vietnam War next door, the monarch was living in exile after being overthrown by Lon Nol. The king then forged an unofficial alliance with the Khmer Rouge, and Mr. Ieng was named special envoy to the royal government.
In 1951, Mr. Ieng married Khieu Thirith, whose sister was the first wife of Pol Pot. Khieu, who served as the Khmer Rouge’s social affairs minister, was a defendant in the genocide proceedings until being declared unfit to stand trial in September because of dementia.
Olsen, the spokesman for the tribunal, lamented of Khieu’s husband: “We are disappointed that we could not complete the proceeding against Ieng Sary.”