In was born in Kampong Trach, Kampot, into a farming family, and was the nephew of In Tam, a provincial governor and prominent Cambodian politician of the post-independence years.
[1] Preap In's uncle, In Tam, is thought to have met with his nephew and offered assurances that In would be given safe-conduct to negotiate directly with Sihanouk in the National Assembly.
[2] Preap In and an associate, Saing San (described as a chauffeur at the Royal Palace) were arrested in Takeo on 19 November 1963; they were brought to Phnom Penh and displayed in cages in public.
[3] Although San was soon to be released, In was brought before the National Assembly, where he readily 'confessed' that his organisation, the Khmer Serei, was funded and controlled by the CIA with the intention of overthrowing Sihanouk.
[5] As a result, Preap In is still remembered in Cambodia as a figure of opposition to the Sangkum: the former Cambodian Prime Minister, Hun Sen, had several times threatened to broadcast the film of In's execution as part of his public disagreements with Sihanouk.