Social affairs minister under the Khmer Rouge, Ieng Thirith was arrested Monday with her husband, Ieng Sary, the deputy prime minister and foreign minister of the communist regime blamed for the death of some 1.7 million people in the late 1970s.
Deeply entwined in the group's leadership, she was the sister-in-law of Pol Pot, the top Khmer Rouge leader who died in 1998. Her sister, Khieu Ponnary, Pol Pot's first wife, died in 2003.
Her birth date is uncertain, though according to a document filed by tribunal prosecutors, she was born in Battambang province in northwestern Cambodia around 1932.
Given the name Khieu Thirith, she was the daughter of a provincial judge, allowing her and her sister lives of relative privilege. She adopted Ieng Sary's surname when they married, as students, in Paris in the 1950s.
Ieng Thirith was among the first Cambodian women to achieve academic prominence, graduating in English literature in Paris, then working as a professor after returning to Cambodia in 1957. Three years later she founded a private English school in the capital Phnom Penh.
But at the same time she was a member of an underground circle of Cambodian leftists, and she followed her husband into the jungle to flee government repression in 1965. The communist movement then became a guerrilla force that triumphed over the pro-American government in 1975, putting Pol Pot and the Khmer Rouge into power.
As minister of social affairs, Ieng Thirith toured Cambodia 's northwestern region in 1976 to investigate health conditions, which she reported - accurately - were disastrous.
Reportedly "shocked" by what she saw during her trip, she told Pol Pot that "foreign agents were infiltrating our ranks" to undermine the revolution, according to research done by the Documentation Center of Cambodia, an independent organization that gathers evidence of Khmer Rouge atrocities. The center is not connected with the tribunal.
Based on Ieng Thirith's report, Pol Pot ordered a purge of Khmer Rouge cadres in the northwest whom he considered enemies of the revolution - one of several bloody episodes reflecting the regime's extreme paranoia and inability to recognize the problems its own policies were causing, according to the documentation center.
Those purged were arrested and taken to the Khmer Rouge's notorious S-21 prison in Phnom Penh, according to the prosecutors' filing to the tribunal, a copy of which was obtained by The Associated Press.
Up to 16,000 alleged enemies of the regime were tortured then executed at the prison. Only about a dozen detainees are thought to have survived the prison ordeal.
Ieng Thirith's participation included the "planning, direction, coordination and ordering of widespread purges ... and the unlawful killing or murder of staff members from within the Ministry of Social Affairs," the prosecutors claimed.
After the ouster of the regime in 1979, the Khmer Rouge continued to fight from the countryside.
Ieng Sary defected to the government in 1996, effectively prompting the movement's complete downfall two years later. Since his surrender, Ieng Sary and his wife have lived quietly in Phnom Penh.
Cambodian newspapers recently questioned the state of Ieng Thirith's mental health, reporting that she showed signs of dementia - a condition that could ensure she is considered unfit to stand trial.