(CNN) -- It was one of the worst genocides since the Nazi era. The brutal Khmer Rouge took over Cambodia in 1975 and terrorized the population for four years, killing more than one million people.
One of its infamous leaders died Thursday, escaping judgment for war crimes at the hands of a U.N. tribunal.
Ieng Sary passed away in the capital Phnom Penh at age 87, the United Nations-backed court for Cambodia said.
He was the foreign minister under, and the brother-in-law-of, Khmer Rouge dictator Pol Pot.
On Tuesday, he was hospitalized for stomach pains.
The Extraordinary Chambers in the Courts of Cambodia -- the U.N.-backed court -- will establish the cause of death before releasing his body to his family.
Ieng Sary's death leaves just two defendants facing judgment by the tribunal.
His wife, Ieng Thirith, was excluded from the case after she was deemed unfit to stand trial in September 2012.
But charges of torture and murder will continue against the remaining defendants -- former nominal head of state Khieu Samphan and former prime minister Nuon Chea.
Following Ieng Sary's death, Amnesty International asked the court to continue with its proceedings without delay.
"The news of Ieng Sary's death will be difficult for the victims of the Khmer Rouge crimes who have waited so long for justice," said Rupert Abbott of the human rights group.
The court has faced criticism that it has proceeded too slowly with the trial. The defendants are more than 77 years old, and more of them could die before facing judgment.
On Tuesday, proceedings came to a grinding halt after local staffers walked out, saying they will not return until they are paid wages owed by the cash-strapped court for the last three months.
A bloody period
Pol Pot came to power as a communist revolutionary after toppling the U.S. backed government in Phnom Penh, when American troops left the region in 1975 at the end of the Vietnam war.
For the next four years, the communist Khmer Rouge under Pol Pot was blamed for the deaths of some 1.7 million people through torture, execution, hard labor and starvation.
Pol Pot's rule became known as the era of the killing fields when cities were emptied and residents slaughtered.
A Vietnamese invasion in 1979 forced the Khmer Rouge to retreat back to the jungle.
The dictator died in 1998 before the tribunal's existence and never faced trial.
The Vietnamese who invaded Cambodia quickly put Ieng Sary on trial and convicted him in absentia in 1979. When he defected from what remained of the broken Khmer Rouge regime in 1996, the Cambodian king pardoned him.
The defense for Ieng Sary once argued that the case against him should be dismissed on grounds of double jeopardy -- a defendant cannot be tried twice for the same crime.
The court considered the motion.
CNN's Sara Sidner contributed to this report