The former leader of the brutal Maoist regime in Cambodia has died, dashing hopes that the man allegedly responsible for millions of deaths would be punished.

Ieng Sary was on trial for crimes against humanity, war crimes and genocide committed under the Khmer Rouge regime at the time of his death on Thursday. He was 87.

Ieng Sary founded the Khmer Rouge with leader Pol Pot, his brother-in-law, and served as the regime’s foreign minister after overthrowing a US-backed regime.

He defected from the Khmer Rouge in 1996 as it began to fall apart, and was granted a pardon, BBC reports. He was arrested in 2007 when an international tribunal was established.

A spokesman for the tribunal confirmed Ieng Sary’s death. “We are disappointed that we could not complete the proceeding against Ieng Sary,” Lars Olsen said.

Two other former Khmer Rouge leaders – Nuon Chea and former head of state Khieu Samphan,  both now in their 80s – were being tried by a joint Cambodian-international tribunal for war crimes.

The Communist regime, which ruled Cambodia from 1975 to 1979, claimed it was building a pure socialist society by forcing people 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.