In Mike Rodelli's latest book In the Shadow of Mount Diablo he makes it obvious that he believes the SLA and Red Phantom letters are communications mailed by the Zodiac Killer. He is also fairly confident that the true Zodiac letters were not licked by the sender. This being the case, then he must accept that the SLA and Red Phantom letters should have no oral epithelial cells on them. The Badlands postcard was pre-stamped, so provided no opportunity for reliable DNA testing. Therefore, the only possible match between the DNA from the 1978 letter and one of the 1974 letters, using Mike Rodelli's thought process, had to be between the 1978 letter and the Exorcist letter. If Alan Keel's claimed statement is correct, then I agree. The problem however, is that the Exorcist letter is almost certainly genuine Zodiac material because of the comparison between its envelope and the unpublished envelopes from April 1970 (which hadn't been published by January 29th 1974). So, if the Exorcist letter is genuine, so is the 1978 letter.
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THE GENETIC IDENTITY OF THE KILLER WE MAY ALREADY HAVE ZODIAC DNA