Prosecutors in the U.N.-backed trial of three top Khmer Rouge leaders in Cambodia accused the men of masterminding one of the worst horrors of the 20th Century.

In opening statements Monday, prosecutor Chea Leang said Khmer Rouge head of state Khieu Samphan, reputed chief ideologue Nuon Chea and Khmer Rouge Foreign Minister Ieng Sary orchestrated the bloody “killing fields” revolution that wiped out a quarter of the population and destroyed the lives of millions of survivors.

The three men are accused of crimes against humanity, including genocide, homicide and torture. The Khmer Rouge ruled from 1975 to 1979, when up to two million Cambodians are estimated to have died.

The U.N. High Commissioner for Human Rights Navi Pillay, welcomed the opening of the trial, saying testimony would help a new generation of Cambodians to understand their history and add stimulus to the international community's efforts to prevent future mass crimes.

Prosecutors are expected to seek the maximum penalty of life in prison for the defendants.

A fourth defendant, Ieng Thirith, was ruled unfit to stand trial last week because she has Alzheimer's disease. She is Ieng Sary's wife and served as the minister for social affairs for the Khmer Rouge.

The Khmer Rouge's supreme leader Pol Pot died in 1998 in a jungle camp, where he was held prisoner after his former comrades turned on him.