Mam Nai was born in Kampong Thom Province in 1934, while Cambodia was under French domination. He was taught by Son Sen at the Institut de Pédagogie in Phnom Penh, becoming a natural sciences teacher in 1956, and Kompong Thom's Balaign College principal in 1958. In the school environment he got to know Kang Kek Iew who was his deputy principal. Both were arrested by Norodom Sihanouk's security services in 1967 owing to their leftist activities. After being freed from jail by Lon Nol in 1970, Mam Nai joined Duch in the guerrilla zone under the control of the Khmer Rouge. He assisted in the interrogation and torture of prisoners at 'M-13 prison camp', the first prison Duch set up in the forests of Amleang, Thpong District. Together with Tang Sin Hean Mam Nai helped Duch to perfect his interrogation techniques in order to purge perceived "enemies of the revolution" from the Khmer Rouge ranks. Prisoners at these camps, mostly disgraced Khmer Rouge cadres, were routinely starved and tortured to extract real and made-up confessions. Few prisoners left the camps alive. Following the Khmer Rouge victory in April 1975 Duch and his men set up prisons throughout the capital including the infamous Tuol Sleng prison. By 1978, as the party paranoia of seeking unchallenged authority and the ensuing purges increased towards the end of Pol Pot's rule, more and more people were brought to Tuol Sleng. Mam Nai was fluent in the Vietnamese language, uncommon among Cambodians, and took part in the interrogation and torture of Vietnamese-background prisoners, contributing to the extermination of the Vietnamese Cambodian minority. Mam Nai's signature is on scores of documents detailing the torture of DK's political opponents. He was together with his boss Duch seeing to the execution of surviving prisoners before abandoning Tuol Sleng prison and both were among the very last Khmer Rouge cadres to flee Phnom Penh when it fell to the People's Army of Vietnam on 7 January 1979. After escaping to the border, Mam Nai joined one of the Khmer Rouge groups that had found sanctuary in Thailand. In the 1990s he was still working as an interrogator for the Khmer Rouge after joining Front 250, commanded by Ny Korn. Mam Nai left the Khmer Rouge shortly before Pol Pot ordered Son Sen's assassination in 1997, living as a private small-scale farmer in the West of the country. Although Cambodian authorities knew where he lived for a long time no attempt was made to arrest him. According to Stephen Heder of London University Mam Nai is implicated in "hands-on torture and execution and would almost certainly be convicted in any international tribunal". After appearing at Duch's trial as a witness Mam Nai has, however, not been charged. At the trial he was asked questions regarding his involvement in the torture and murder of Phung Ton, former dean of Phnom Penh University, but Mam Nai carefully avoided incriminating himself.
