The first interesting aspect of the December 20th 1968 murders was that the bullet fired into David Faraday did not contain the same class characteristics as the other seven submitted bullets. What the Department of Justice report actually said was 'All bullets submitted were Western copper coated .22 long rifle bullets, although some were damaged, it was possible to determine ALL but Item  had 6 right hand groove class characteristics.' Item  was the bullet recovered from David Faraday. This doesn't categorically separate this bullet as being fired from a second weapon, but it certainly opens up the possibility, as it couldn't be definitively linked to the other seven bullets. Furthermore, only nine casings were documented in the police report and the Department of Justice report, when clearly ten casings were ejected from the weaponry that night and explicitly marked in numerous police sketches. So why did this tenth casing, described by Zodiac and depicted in the police sketches, seemingly disappear on its journey from the crime scene to the Department of Justice evidence room. In all likelihood, it is because this casing was the one discovered on the front passenger side floorboard of the Rambler, either left in situ purposely or inadvertently when the Rambler of David Faraday was towed away, but which ultimately became separated from the other nine. Having never been tested, we will never know if this 'missing' tenth casing matches the other nine from a ballistics standpoint. Had it displayed unique characteristics, then there remains a strong possibility that this casing could have been the one ejected from the bullet fired into David Faraday's brain - as this was the only bullet not linked the other seven. This would open up the notion of a second weapon that night, or likely a second shooter - but without the tenth casing being unearthed, this remains just speculation. See Lake Herman Road-The Ballistics Report.
Imagine that two attackers were present at Lake Herman Road, and as inferred above, one was responsible for the close quarter execution of David Faraday, while the other was responsible for the remaining nine shots. In other words, the second shooter had a containing role, securing David Faraday as he left the Rambler. If this were the case, it could be inferred that the assailant who murdered Betty Lou Jensen was the author of the 'Debut of Zodiac' letter and wrote "What I did was tape a small pencel flash light to the barrel of my gun. If you notice, in the center of the beam of light if you aim it at a wall or ceiling you will see a black or darck spot in the center of the circle of light about 3 to 6 inches across. When taped to a gun barrel, the bullet will strike in the center of the black dot in the light. All I had to do was spray them as if it was a water hose; there was no need to use the gun sights". He did essentially "spray" Betty Lou Jensen with five bullets, but he didn't 'spray them', so he could be writing about the events that night from his perspective, not his accomplice's - who may have clinically murdered David Faraday with a single bullet and suggested at the beginning of this article.
Another example of interpretation can be found in the wording on the first three letters mailed on July 31st 1969, where the author stated alongside part one of the cipher "the other 2 parts have been mailed to the S.F. Examiner + the S.F. Chronicle". Alongside parts two and three of the cipher the author stated "the other 2 parts of this cipher are being mailed to the editors of the Vallejo Times and SF Examiner". and uses the present tense of "are being mailed". On all three correspondences the author is tending towards the third person perspective. This is most unlike the Zodiac, who would usually not fail to refer to himself, such as "I have mailed the other two parts of the cipher." Clearly though, this boils down to the subjectivity of the reader and the dissection of such material.
Did these two individuals reappear just over six months later, both of whom were possibly sighted and recalled by Michael Mageau, who survived the horrific onslaught at Blue Rock Springs Park on July 4th 1969. Michael Mageau would recount two significant sightings that night. Firstly, the vehicle that pulled up alongside Darlene Ferrin's brown Corvair - and either this vehicle returning five minutes later to the rear of their Corvair - or a second vehicle now entering the scene, implying that two males may be involved in the crime. Michael Mageau stated in the police report "Shortly after this and about 5 minutes before the shooting occurred, a vehicle pulled into the lot, coming from the direction of Springs Road and Vallejo. The driver turned the lights off on the car and pulled around to the left or east side of their car, approximately 6 or 8 feet away and sat there for a minute. He asked Dea if she knew who it was and she stated 'Oh, never mind'".
The Zodiac Killer appeared bold and brazen, particularly at Lake Berryessa and Presidio Heights, where he took enormous risks beyond what was required to execute his murders. The wearing of an elaborate costume, the writing on the car door, and the murder of Paul Stine in the heart of San Francisco (that included the tearing of Paul Stine's shirt), yet here, if we contend that both these sightings by Michael Mageau were of the Zodiac Killer, then the hesitancy looks out of place. When the first vehicle pulled into the parking lot it was empty, other than Darlene's Corvair. Why not pull up behind the intended victims, as in the second instance, and begin the attack? It could be argued that the first vehicle which pulled up alongside the Corvair was a totally innocent motorist, but the parking lot was totally empty, so why pull alongside the only other vehicle and extinguish the car's lights for upwards of a minute? This could be construed as suspicious activity in the run up to the attack at Blue Rock Springs, in similar fashion to Lake Herman Road - and again - as with Lake Herman Road, this person (if innocent) never came forward as an eyewitness to this crime and has never been traced. Three significant eyewitnesses over three murders that all failed to make themselves available.
It is perfectly feasible that two people were inside Paul Stine's taxicab as it approached the corner of Washington and Cherry just before 9:55 pm. The first person shot Paul Stine and immediately left the scene, taking the 'smoking gun' with him. The second person remained to secure the shirt piece and wipe down the taxicab. Had the second person still been inside the taxicab as police approached, he could just claim to be a witness to the murder or aftermath - and seeing a taxicab driver had been injured or shot, had entered the taxicab to assist the driver. After all, this accomplice would have been devoid of any firearm and incriminating gunshot residue, so how could he be the shooter? The tearing of any shirt (had this been noticed) could be attributed to first aid, acquiring material to staunch the blood loss. However, as police had not arrived in time, any back up plan was not required and the accomplice headed north along Cherry Street.
It would have been an extremely audacious act to have shot a taxicab driver in a built up area, only yards from nearby residences, and then have taken the time to carefully tear a rectangular section of Paul Stine's shirt while still in possession of the only item that would inextricably link you to the crime - the smoking gun. The same could be said of remaining at the scene after the Lake Berryessa double stabbing, and taking the time to write on the car door of Bryan Hartnell's 1956 White Karmann Ghia. Unless, both of these tasks were performed by an accomplice, thereby reducing the risk factor of each crime.
It is actually impossible to disprove that two people were involved in the Zodiac crimes, as it is for multiple co-conspirators. It is equally impossible to prove a lone killer in all four confirmed attacks, as it simply boils down to what you believe. The murderer crafted an elaborate costume for the attack on Cecelia Shepard and Bryan Hartnell on September 27th 1969, but the creation of the 'Zodiac Killer' just prior to August 4th 1969 may have been the greatest disguise of them all.