 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 wasnt 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 (qv), organized the permits for the traffic jam  during the chase at the end of the film.