Nuon Chea, known as "Brother Number Two," below only Pol Pot, passed away on Sunday.
According to AFP, the cause of death was not announced, though Chea had been in the hospital since last month.
Chea was considered the Khmer Rouge's chief ideologue, and though he played a major role in the group's rule, which left roughly two million Cambodians dead between 1975 and 1979, he was not arrested until 2007.
He was sentenced to life in prison last year in a trial at the Extraordinary Chambers in the Courts of Cambodia (ECCC), the United Nations-backed court. The news source reports that Chea had been found guilty of genocide against ethnic Vietnamese and the Cham Muslim minority group.
This was Chea's second life sentence; the first was handed down in 2014 for his role in the forced evacuation of Phnom Penh in 1975.
The ECCC was created in 2006 in an effort to serve justice to senior Khmer Rouge leaders, but only three people have been convicted thus far. Pol Pot, the leader, died in 1998.
According to Time, Chea never apologized for his prominent role in genocide. He said during his trial that he and his peers weren't "bad people," and he denied any responsibility for deaths.
In 2011, he told the court: "I don't want the next generation to misunderstand history. I don't want them to believe the Khmer Rouge are bad people, are criminals. Nothing is true about that."
It is expected that the unrepentant Chea will be the last ECCC trial.
[Photo via Time]
