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Several contemporary news items noted that in February 1942, Republic purchased an original story by M. D. Christopher entitled "London Blackout Murders." Los Angeles Times reported that the story detailed the "activities of a killer during the periods of darkness brought about by the war," while Hollywood Reporter asserted that the story was about "recent femme murders," and would be turned into a screen treatment by Christopher. The extent of Christopher's contribution to the completed film has not been confirmed, however, as the Screen Achievements Bulletin and reviews list only Curt Siodmak as the picture's writer. Later Hollywood Reporter news items noted that the film was to be "based on recent headlines concerning a killer who ran amuck in the British capital," and that the studio intentionally cast lesser known players in order "to befoozle the ticket buyers who is the hero, the heavy, the ingenue, etc." The news items quoted above May have referred to the case of R.A.F. cadet G. F. Cummins, who, in March 1942, was charged with murdering four women and attempting to kill two others. After being convicted of one of the murders, Cummins was sentenced to death. The case did involve "femme murders," and May have been the initial inspiration for the picture, but the facts of Cummins' crimes bear no resemblance to the finished film.
Although a Hollywood Reporter news item and a production chart include Hugh Huntley and Charles Irwin in the cast, their participation in the finished picture has not been confirmed. Several reviews incorrectly list Morton Scott as the film's music director.