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The attack depicted in Greene's novel resembles the terrorist attacks on the opera house and city hall in Saigon on January 9, 1952. Initially attributed to the communists, the bombings were the work of Trình Minh Thế's faction.
Greene suspected American involvement, and was told by the French commander in Vietnam, Raoul Salan, that an American diplomat had been arrested during this period in the possession of plastic explosives. The journalist Jean Lartéguy reported that French investigators concluded that the US had supplied Trình Minh Thế with detonators and bomb timers during the same period.
Source: Davis, M. (2017). Buda's Wagon: a brief history of the car bomb. Verso Books.
There does not seem to be reason to doubt the thrust of these allegations that the US supplied Trình Minh Thế with bombmaking materials during the relevant time period.