Vallejo Chief Jack E. Stiltz stated "he is still not convinced the letters and codes were written by the actual killer although the letters contained details of the crimes that are not public knowledge. The letter writer described the clothing of the victims, position of the bodies, and even the brand names of the ammunition used. Such a knowledge could have been obtained from any witness at the death scenes." He urged the writer to send in more facts to prove his connection to the crimes. The fact that the killer responded immediately on August 4th 1969 with "In answer to your asking for more details about the good times I have had in Vallejo, I shall be very happy to supply even more material," and introducing himself as the 'Zodiac', is testimony to how this newspaper shaped the very nature of the Zodiac case.
Had Jack E. Stiltz not "urged" the killer for more material, it is highly likely this letter would not have been sent - and in view of the absence of any Lake Berryessa communication, the murderer of five would have been just that on October 11th 1969. The serial killer under the pseudonym Zodiac would very likely not have existed. Additionally, this would have left just one set of communications, the July 31st 1969 trinity of letters, the only correspondence mailed by the killer spanning his five confirmed murders. Subsequent to October 11th 1969, the Zodiac Killer mailed copious letters and cards, with no confirmed further attacks. This brings us on to the modus operandi of the Zodiac Killer.
Some people claim that the Lake Berryessa attack was not the Zodiac Killer based on M.O. Could a case ever be made that the Zodiac Killer had a pattern or M.O. after only two attacks at night? Why shouldn't the Zodiac Killer have worn a hooded costume, bearing in mind this attack was perpetrated in daylight hours in a recreational setting? The argument for an M.O. based on just Lake Herman Road and Blue Rock Springs appears rather groundless, when we consider that no eyewitnesses survived the first attack and the limited sample size of the two known crimes to date.
The fact that he used a knife instead of a gun at Lake Berryessa as his primary weapon of attack, is also used to discredit any Zodiac involvement. This argument is equally facile. From what we are led to believe, by the evening of September 27th 1969, he had used a knife in 33.33% of his attacks, yet he had attacked couples 100% of the time. Why does the 'knife' argument supersede the 'couple' argument in discrediting this as a Zodiac crime. The proponents of a Lake Berryessa copycat will concede that the M.O. of the Presidio Heights murder would have been equally questioned, had it not been for the subsequent letters and shirt pieces mailed by the killer. If not for this corroborative proof, his third and fourth attacks would have been wholly dismissed, based on just the first two crimes. Why is the Zodiac Killer compelled to stay rigid in his attacks forevermore, subsequent to July 4th 1969? Any deviation whatsoever, must by inference, be regarded as grounds to rule it out. The assumed deviation of Lake Berryessa based on M.O. is weakened and diluted as an argument, when the shooting of Paul Stine occurred in a built-up area in San Francisco, perpetrated inside of a taxicab, on a lone 29-year-old male, with 'souvenirs' taken from the crime scene. If either the Modesto abduction of Kathleen Johns or Lake Tahoe disappearance of Donna Lass were confirmed as Zodiac crimes, then the M.O. of the Zodiac Killer based on just the first two attacks is made even weaker as a credible argument.
One of the strangest arguments you will encounter, is that the Lake Berryessa attack was perpetrated by a copycat killer. Many reasons will be put forward as to why this wasn't the Zodiac Killer, such as the weapon of choice (the knife), the executioner's costume, the writing on the car door not looking like Zodiac's handwriting, and the fact the perpetrator of the crime didn't identify himself as Zodiac. Then he wasn't really a very good copycat was he. This person supposedly went out of their way to impersonate the Zodiac Killer, but failed to be a copycat. And if he wasn't a very good copycat, it is highly likely he wasn't trying to be one - because he was the Zodiac Killer. You cannot claim this crime not to be Zodiac, because it was so different to Lake Herman Road and Blue Rock Springs, while claiming the responsible at Lake Berryessa to be a copycat. The two are diametrically opposed.
A modus operandi or MO refers to the method or procedure that a criminal uses when committing a crime. It comes from the Latin phrase meaning “mode of operation”. Basically, all criminals have a method of operation that encompasses the habits, techniques, and any peculiarities of behavior. The method can stay the same, but it will often grow and change over time as the criminal becomes more skillful at committing a specific crime. authorjenniferchase.com.
Can two crimes really be considered a habit? If we take the Lake Berryessa crime away from the Zodiac story, can we confidently claim the Zodiac Killer had formed a habit encompassing Lake Herman Road and Blue Rock Springs, separated by nearly seven months? If we discount the Lake Berryessa attack, then what little method of operation the killer had, has literally vanished - just like he did nearly fifty years ago.