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The top surviving members of the brutal Khmer Rouge regime are facing justice for the first time Monday, standing before a U.N.-backed tribunal in Cambodia's capital, Phnom Penh. Their trial comes three decades after their plans for a communist utopia in Cambodia left an estimated 1.7 million people dead by execution, medical neglect, overwork and starvation. A look at the four:

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Nuon Chea, 84. The Khmer Rouge's chief ideologist, known as "Brother No. 2." He was second in command to Pol Pot, the top leader who died in 1998. He is accused of crimes against humanity, genocide, homicide, torture and religious persecution. He denies guilt and says he is not a "cruel" man but acted as a "patriot."

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Khieu Samphan, 79. The regime's former head of state. He is accused of crimes against humanity, homicide, torture and religious persecution. He has denied responsibility for the atrocities and blames Pol Pot for the group's policies. In his 2004 memoir, he says he was only a "shell" for the Khmer Rouge and had nothing to do with its radical policies.

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Ieng Sary, 85. The Khmer Rouge's former foreign minister. He is accused of crimes against humanity, genocide, homicide, torture and religious persecution. He disputes the charges and has demanded his guilt be proven. Ieng Sary and his wife were part of Pol Pot's inner circle that made key decisions. He is accused of persuading diplomats and intellectuals based overseas to return to Cambodia. Most of the returnees were executed.

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Ieng Thirith, 79. The wife of Ieng Sary, a sister-in-law of Pol Pot's and the regime's minister of social affairs. She is accused of involvement in the "planning, direction, coordination and ordering of widespread purges" and has been charged with crimes against humanity, genocide, homicide, torture and religious persecution. She has called the charges "100 percent false."