Robert Prouty joined the San Diego Police Department after his service in the U.S. Air Force, before relocating to Sacramento in 1973 and working for the Department of Justice as a Questioned Documents Examiner. In the San Francisco Examiner on October 30th 1987, he stated in reference to two potential Zodiac letters "My very firm opinion is that they were not written by the authentic Zodiac. They appear to be composed of previous letters which have since been made public. All of us were hoping we'd heard the last of him", said Prouty, who analyzes about fifteen bogus Zodiac letters a year.
If we take the word of Robert Prouty, then he must have examined a minimum of 222 communications alleged to have been mailed by the Zodiac Killer over those nearly fifteen years.. Even if 20% of these were genuine, the Zodiac Killer may have mailed at least 44 communications from 1973 to October 1987. With a high degree of probability the Zodiac Killer mailed at least four, if not five communications in 1971. We therefore have a potential 222 communications ruled as bogus, when in fact some of them may have been genuine Zodiac letters - and as such - provided invaluable information as to his identity, or further insight into his movements and character? Hoping that you've "heard the last of him" is hoping for less information about the killer. More disturbing still, is how many of these communications have ended up being discarded and destroyed, when they could have contained valuable DNA evidence at a time when major breakthroughs in biological science were surfacing, along with the advancement in preservation and storage of samples pertaining to criminal investigations.
In respect to the 1987 letter received by the Vallejo Times-Herald, Robert Prouty said it "appeared to be composed of previous letters which have since been made public". In other words, it had been copied from previous letters and was a case of mimicry. Here is what Robert Prouty stated in respect to the 1978 letter: "I examined the photographs of the April letter and those of previous letters attributed to Zodiac. My first impression was that it was in the same general style as previous letters, but after closer examination my ultimate conclusion was that there were so many differences that it was not written by the same person who wrote the previous Zodiac letters … Several letter characteristics in my opinion did not match the style used by Zodiac. The slant of some letters was not consistent with previous Zodiac communications". In summary, one was ruled out because it looked too similar to previous letters and one was ruled out because it looked too different. John Shimoda of the US Postal Service crime laboratory originally deemed the 1978 letter genuine, before reversing that decision and stating “I am of the opinion that the letter of April 24 was an attempt to duplicate Zodiac letters and is not authentic".
The Zodiac Killer contacted the Vallejo Times-Herald on July 31st 1969 in his first contact with the newspapers, but according to the Vallejo Times-Herald City Editor Mary Apanasewicz on October 30th 1987, the newspaper had never received a crank letter in the previous eighteen years. Therefore, we would have to conclude that up to 222 random hoax Zodiac letters could have been mailed between 1973 and 1987, yet not one single individual hoaxer ever wrote to the Herald. The fact that somebody wrote to them on October 28th 1987, using the same thirteen words on the envelope as the July 31st 1969 envelope, with no punctuation in either, using the full title of Vallejo Times-Herald (with both likely misspelling Herald to Herold), when the 1969 envelope had not been released into the public domain by October 28th 1987, should have told Mary Apanasewicz that she still hadn't received a hoax letter by the end of 1987 either.