 Writer Hans Janowitz (qv) claims to have gotten the idea for the  film when he was at a carnival one day. He saw a strange man lurking  in the shadows. The next day, he heard that a girl was brutally  murdered there. He went to the funeral, and saw the same strange man  lurking around. He had no proof that the strange man was the  murderer, but he fleshed the whole idea out into his film.