The first claim is that the modus operandi (MO) of the crime was totally different from the other three attacks. This is clearly not true. It was an attack on a young couple, seemingly random, just like the first two crimes preceding it, despite the sample size being relatively small and statistically weak. Had the killer not mailed pieces of Paul Stine's shirt with three subsequent letters or written to the newspapers, neither the police or anybody else would have likely connected his murder to the Zodiac Killer. The modus operandi would have been called into question. So claiming the Lake Berryessa crime as not that of the Zodiac Killer because it didn't marry up perfectly with the first two crimes, is again, not evidence of anything. Absence of perfect continuity is not evidence to the absence of Zodiac.
The message on the car door detailing his crimes to date shows distinct Zodiac traits in the handwriting, but expecting it to match up perfectly with his mailed correspondence is not a reasonable premise. He had just stabbed a couple sixteen times, his adrenaline was likely raised, he had then traversed back up the hill and proceeded to write on the car door of Bryan Hartnell's car, on a different surface, in larger handwriting, in a totally different position he would adopt if he were writing calmly on a desktop. The comparison between the two must take this into account, so the fact it looks somewhat similar to his mailed correspondence is actually quite an achievement. Had this been written by a copycat it would actually have been a lot harder, who not only had all of the above to contend with, but had to replicate the handwriting from an awkward stance or seated position, side on to the vehicle.
It is possible a copycat read the transcript of the Zodiac Killer's first payphone call to police in the newspapers and reproduced its phraseology during the Napa payphone call, but why is it easier to believe that, rather than believing it was the Zodiac Killer? What evidence exists to promote a copycat making the call above the Zodiac Killer? There is none. The same can be said of the handwriting on Bryan Hartnell's Karmann Ghia.
In terms of the content of the handwriting, it is often said it was not the Zodiac Killer because his November 8th 1969 'Dripping Pen' card made reference to a murder or possible double murder in August, which the killer at Lake Berryessa did not refer to on the car door. But the reverse could be argued, that the reference to September in the 'Dripping Pen' card is equal proof that he was claiming the Lake Berryessa crime as his, particularly in light of no other valid murders in September where a killer wore a crossed circle costume and made a phone call with all the hallmarks of Blue Rock Springs Park. This again is a false argument. Even if the Zodiac Killer had murdered in August, the police were apparently unaware of this fact prior to the Lake Berryessa attack, so he simply wrote the dates of the crimes he knew the police would recognize.
At the top of the Karmann Ghia door the Zodiac wrote 'Vallejo,' so any murder outside of this area would have been unnecessary to include in the message. This indicated that the Zodiac believed that both the Lake Herman Road murders and the attack at Blue Rock Springs Park were both committed in the jurisdiction of Vallejo. This is understandable, because the jurisdictional line that separates Benicia and Vallejo is a short distance from the Lake Herman Road turnout. The Zodiac never mentioned Benicia in any of his correspondence to the newspapers, however, he did refer to Vallejo several times. Curious then, that a copycat would make the same mistake and lump the 12-20-68 on the car door under the banner of Vallejo also. If this alleged copycat had read the newspaper transcripts of the Blue Rock Springs Park phone call, he certainly didn't take the time to read the multiple references in the newspapers to Benicia when referring to the first crime on the car door. It could be argued that he studied the Zodiac letters intently and knew the tendencies of the Zodiac Killer, or we could opt for the more likely reasoning, that he was the Zodiac Killer.
Much debate has occurred on whether the man spotted by the three girls at Lake Berryessa was the assailant of Bryan Hartnell and Cecelia Shepard, with comparisons made of this man to the Presidio Heights sketch. Many have stated the two artist impressions are dissimilar in appearance, yet with a quick hair change and slight alteration to the eyebrows, the difference is much less significant between the two. Eyewitness recollections are also proven to be notoriously unreliable. These hair changes have been made to the Lake Berryessa sketch below, without altering any of the facial features.
In an ABC Primetime documentary the narration was as follows "However if Doctor Cydne Holt can find enough genetic material from Zodiac's stamps and letters, she can compare it to a wafer thin slice of brain tissue from Arthur Leigh Allen's autopsy." Dr Cydne Holt "This brain tissue from Arthur Leigh Allen is the reference sample that I would use for the comparison." Narrator "Dr Holt has already detected the possible presence of Zodiac's DNA in the seal of the envelope that contained the greeting card (Dripping Pen Card and 340 cipher), and just in case that test fails to provide a full DNA profile, she also prepares to look for DNA beneath the stamps on two of these three letters (July 31st letters)." Dr Cydne Holt "Depending on whether those DNA's match each other, might allow me to include or exclude Arthur Leigh Allen as potentially contributing the DNA on the Zodiac letters." She was able to exclude Arthur Leigh Allen as the contributor, indicating a positive match was found between the letters. So if we maintain that the 'Dripping Pen' card and August 31st letters were mailed by Zodiac - and he claimed September as his crime - it lends credence to the Lake Berryessa crime being committed by Zodiac (this DNA has subsequently been questioned).
The Zodiac Killer probably believed that by writing on the car door and making the telephone call, it would be enough for people to conclude he was the killer. However, this has not proved to be the case. He probably believed that mailing a section of Paul Stine's bloody shirt would be proof enough. This has also failed in certain quarters. The details only known to him and the police at Lake Herman Road and Blue Rock Springs Park he thought would be enough, but it wasn't. The reality: is that for some people there is nothing he could have done to prove he was the killer. If it's claimed that this crime had a totally different modus operandi to the first two crimes and was therefore not the Zodiac Killer, then the copycat was obviously not doing a particularly good job. A copycat attempts to replicate the crimes of a known killer. Therefore, we could conclude that by failing to stick to the known MO at Lake Herman Road and Blue Rock Springs, the Lake Berryessa perpetrator was more than likely the Zodiac Killer after all. But the fact remains, that not one piece of tangible evidence points to a copycat killer, only the idea that such a person exists.