PHNOM PENH, Cambodia — They were neighbors, two elderly retirees tending their gardens and playing with their grandchildren, hoping that the world would “let bygones be bygones” and forget they had been leaders of the murderous Khmer Rouge regime.

But their retirement did not last. Eleven years after their arrest, and after a long and expensive trial, they stand as the only surviving members of a tight-knit Communist leadership to be held responsible for the killing of at least 1.7 million of their countrymen from 1975 to 1979. On Friday, a tribunal found them guilty of genocide.

One of them, Khieu Samphan, 87, was once an admired, incorruptible schoolteacher and member of Cambodia’s Parliament, who fled arrest for his leftist views in the 1960s and joined the young insurgent movement in the countryside. Suave and multilingual, he later became the international face of the Khmer Rouge as its nominal head of state.

The other, Nuon Chea, 92, the movement’s ideologue, was perhaps the truest believer in its attempt to turn Cambodia into an agrarian utopia, killing off its educated people and reorganizing the country into what amounted to a nationwide labor camp. Known as “Brother No. 2” to the late Khmer Rouge leader, Pol Pot, Mr. Nuon Chea had command responsibility over a wave of murderous purges. He later assured an interviewer that “we only killed the bad people, not the good.”