Son Sen (12 June 1930 – 15 June 1997), alias Comrade Khieu (សមមិត្តខៀវ), was a Cambodian communist politician and soldier. A member of the Central Committee of the Communist Party of Kampuchea/Party of Democratic Kampuchea, the Khmer Rouge, from 1974 to 1992, Sen oversaw the Party's security apparatus, including the Santebal secret police and the notorious security prison S-21 at Tuol Sleng.
Son Sen was responsible for ordering the massacre of more than 100,000 in the Eastern Zone of Cambodia during the last six months of 1978.
Sen was married to Yun Yat, who became the Party's minister of education and information. Along with the rest of his family, he was killed on the orders of Pol Pot during a 1997 factional split in the Khmer Rouge.
Son Sen was born in the village of Huong Hoa, Trà Vinh Province in southern Vietnam to a minor landowning family. Although the Khmer Institute described Sen as being of "Sino-Vietnamese" descent, Béréziat and others have confirmed that like several other Khmer Rouge leaders he was of Khmer Krom ethnicity. From 1946 he attended a teacher training college in Phnom Penh, and in the 1950s received a scholarship to study in Paris, where he became a member of a Marxist group of Cambodian students centred on Saloth Sar (Pol Pot), Ieng Sary, and Hou Yuon. Along with other members of the group, Sen was influenced by the radical line pursued by the French Communist Party.
Sen's academic record was relatively mediocre, and in May 1956 the authorities withdrew his scholarship due to his continued participation in political activity. Returning to Cambodia with a teaching certificate, he taught for a time at the Lycee Sisowath, and went on to become director of studies of the National Teaching Institute, part of the University of Phnom Penh. By 1960 he had also, however, joined the clandestine Cambodian Communist Party, then known as the Khmer People's Revolutionary Party and led by Tou Samouth.
After the Cambodian coup of 1970, and the subsequent establishment of the Khmer Republic under Lon Nol, Sihanouk was to join with his former Communist enemies in forming the GRUNK, a Beijing-based government-in-exile. The Khmer Rouge's forces were soon to be swelled by thousands of new recruits, attracted in part by Sihanouk's name. Son Sen, whilst not a prominent member of the GRUNK, had considerable power thanks to his high position in the government's forces on the ground – CPNLAF, the Cambodian People's National Liberation Armed Forces. Along with Ieng Sary, he was responsible for the North-Eastern Zone, the stronghold of the Party 'Centre'.
By 1972 Son Sen had become Chief of Staff of the Khmer Rouge forces. Despite occasional criticisms – his colleagues found his style peremptory and his point of view "bourgeois", and he was openly criticised by Hou Yuon at the Party's 1971 Congress – his devotion to Saloth Sar and his closeness to the Party 'Centre' guaranteed his senior position.
After the Khmer Rouge took Phnom Penh in April 1975, Son Sen was appointed Deputy Prime Minister with responsibility for Defense. Responsible for internal security, he also oversaw the Santebal – the Khmer Rouge secret police. As such he monitored the operations of the infamous S-21 prison at Tuol Sleng, and engaged actively in the design of its interrogation and torture procedures. Son Sen's role meant that he was particularly closely implicated in the many thousands of deaths that occurred in this period due to the arrest and execution of perceived enemies of the regime. His memoranda to his subordinate Kang Kek Iew, and notes taken during study sessions overseen by Sen for S-21 cadres, reveal his continued interest in history, deeply anti-Vietnamese views, and revolutionary zeal, along with a "schoolmasterish attention to detail". These were combined with a brutally ruthless streak; Sen was reputed to have once been so angry with Foreign Minister Ieng Sary that he unsuccessfully attempted to have him murdered.
The Party's paranoia with regard to security, and its obsessive secrecy, led many senior Khmer Rouge cadres to be identified in internal documents by pseudonyms or numbers: Son Sen was often referred to as "Khieu", and his wife as "At". He was also identified as "Brother 89" on letters and memoranda.
Sen's other main duties in this period involved reorganising the CPNLAF forces into a cohesive national army, the Revolutionary Army of Kampuchea. By 1977–78, he was overseeing an increasing series of clashes with Vietnamese forces along the border, as well as mounting a massive purge of Eastern Zone cadres considered to have Vietnamese links. As the ongoing war with the Vietnamese began to go increasingly badly for the Democratic Kampuchean forces, however, Sen began to fall under suspicion himself, and may well have become a victim of his own security apparatus if the 1979 Vietnamese invasion had not intervened. In particular, he had been implicated by the Party's internal security in several "traitorous" activities including the 1978 murder of the British academic Malcolm Caldwell.
Following the Paris Peace Agreements of October 1991, Son Sen and Khieu Samphan traveled to Phnom Penh to negotiate with UNTAC and the Cambodian government in Phnom Penh. Sen was made a member of the Supreme National Council which was set up to protect the country's independence until UN-supervised elections could take place, and seems to have been more determined than most Khmer Rouge leaders to ensure that they were reintegrated into normal national politics. However, as a result Sen was removed from power in May 1992 by Ta Mok, after a dispute with fellow Khmer Rouge leaders over whether to continue the negotiations. From 1992 to 1997 he had relatively little influence within the Khmer Rouge.
He was murdered on 15 June 1997, alongside 13 members of his family, including children, on orders of Pol Pot, who at the time was fighting his last battle to regain control of the Khmer Rouge from Ta Mok. Pol Pot is thought to have believed that Sen was in negotiations with government forces to surrender, specifically being in contact with then-Second Prime Minister Hun Sen; he ordered Son Sen and his family to be shot, after which trucks drove over their bodies.