The heroin processing lab was built by the Corsican mafia, and was so realistic that the entire set had to be guarded by French police when it wasn't in being used by the film crew. The mafia also advised on the methods used by drug smugglers to get heroin in the US (concealing the drug in freighter weights) and, according to
John Frankenheimer, organized the permits for the traffic jam during the chase at the end of the film.
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