In 2002, a new round of DNA testing was performed on some of the Zodiac letters by Dr. Cydne Holt, yet despite an ABC Primetime documentary opening up with the promising statements of "But today thanks to new crime scene technology there's reason to believe time may have run out on Zodiac. Somewhere, perhaps on the back of this postage stamp or in the seal of this envelope, may be microscopic clues that will now finally lead police to the killer", it appears the early optimism of a Zodiac breakthrough has soured. Dr. Cydne Holt stated "We are hoping to be able to get some genetic information about the individual or individuals that did seal these envelopes or place stamps on them". 

It is patently obvious by these statements the importance of securing genetic material from the sealed sections of the stamps and envelopes. Any testing performed on the outside of the stamps and envelopes is absolutely meaningless from a standpoint of identifying the person who was present at the time of their sealing. Since this documentary aired, it is now claimed that this is exactly what transpired in the laboratory - with the letters being swabbed from the outside. The only reason to perform such a pointless procedure, would be if you had failed to find any or enough saliva on the sealed section of the envelopes or stamps tested, yet were determined to carry on with the 'show'. In other words, the ABC Primetime documentary ended up becoming a circus sideshow for ratings, rather than applying the correct procedure which was promised in the opening statements of the documentary. This led to previously ruled out suspects getting a new day in the sun. Despite the fact this round of testing has been routinely discredited by many in the Zodiac community, the documentary has inadvertently revealed so much more about the Zodiac case. 

The San Francisco Police Department DNA report of communications tested in the late 1990s, indicates that "few cells" were found on any of the Zodiac communications up to and including the June 26th 1970 'Button' letter. The letters subsequently tested after this date were all labelled as "cells found". The Kathleen Johns, Little List, Exorcist and 1978 letters all contained potentially measurable genetic material.

​Even Criminalist Alan Keel (according to Mike Rodelli) revealed: "In contrast (to earlier communications), Keel analyzes two other letters, one of which is the 1978 forgery, and finds that this letter and one of the 1974 letters are loaded with saliva and cells. He then easily extracts DNA from both of these letters using the more primitive DNA technology of that time and finds that the DNA matches between those two letters, thus proving that one person sent both."  The only other letter in 1974 processed for DNA (where cells were found), was the Exorcist letter. Therefore, the two letters described by Alan Keel "as matching between them" had to be the 1978 letter and the Exorcist letter. If the 1978 letter is 'not an authentic Zodiac letter', then the Exorcist letter is 'not an authentic Zodiac letter' either.​

So, here we have four communications that provided notable results, but none were featured in the 2002 ABC Primetime documentary. The fact that the 1978 letter or Exorcist letter were apparently never tested in the documentary, despite being "loaded with saliva and cells", should be enough evidence to conclude that this round of testing didn't consider these two letters as reliable Zodiac communications. Whereas the majority of Zodiac sleuths consider the Exorcist letter to be genuine Zodiac material, the findings of Alan Keel and the reluctance to use this letter at all in the documentary, seems to reject this premise. The documentary made specific reference to the July 31st 1969 letters, the October 13th 1969 Paul Stine letter and the November 8th 1969 'Dripping Pen' card, all of which apparently contained "few cells" according to the San Francisco Police Department DNA report only two years earlier, but made no mention of the July 24th 1970 Kathleen Johns letter or the July 26th 1970 Little List letter, both of which were labelled as "cells found". If you were to choose letters for testing, surely you would choose ones that contained more cells on the sealed sections of the envelopes and stamps. 

Despite the controversy this documentary has generated, it did clearly state "Dr. Holt has already detected the possible presence of Zodiac's DNA in the seal of the envelope that contained the greeting card, and just in case that 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 1969 letters)." 

They would ultimately claim a partial DNA fingerprint which they used to rule out Arthur Leigh Allen, Kjell Qvale and Charles Collins as the infamous Zodiac Killer. But many believe they failed to find enough material from the sealed sections of the communications and thus resorted to swabbing the outside of the envelopes and stamps. However, Dr. Cydne Holt did affirm that she needed to find DNA matching between the envelope seal of the greeting card and the July 31st 1969 letters, stating "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". The act of later ruling out all three suspects, including Arthur Leigh Allen, implied she had found a DNA match between these two separate correspondences. Even if she had swabbed the outside of the letters to keep the 'show' on the road, it would still imply (using her words) that a match between separate communications must be found before ruling Arthur Leigh Allen in or out as "contributing the DNA on the Zodiac letters". But what are the realistic chances of finding a DNA match between the Dripping Pen card envelope and one of the July 31st 1969 letters by swabbing the outside of each correspondence? The concluding elimination of the suspects near the finale of the show seemed to suggest, one way or another, she did. The subsequent reluctance of the participants in the documentary to confirm the validity of the testing and the new round of testing in 2018, appears to negate the ABC Primetime documentary as a reliable venture. 

But here we are again in 2019, waiting on results from the latest attempt to secure a viable genetic fingerprint of Zodiac to submit to GEDmatch. "Vallejo police Detective Terry Poyser, who has worked the Zodiac case for four years, said his agency has submitted two envelopes that contained letters from the Zodiac Killer for a type of advanced DNA analysis that previously had not been available in the case. Poyser declined to identify the lab, but said it would attempt to obtain a full DNA profile from saliva on the envelope flap and stamps. He said he expected to have results back from the lab as soon as in the next few weeks, and almost certainly by summer. The department has three letters and two envelopes from the Zodiac, Poyser said. The envelopes each have a double stamp, which Poyser said was a trademark of the Zodiac. They originally were sent to the San Francisco Chronicle and the San Francisco Examiner, he said. The third letter was sent to the Vallejo Times-Herald, but does not have an envelope with it, he said. Allen largely was cleared as a suspect in 2002, when investigators obtained a partial DNA sample from the envelopes that didn't match his. Poyser said that DNA profile only had a few markers on it, less than half of the genetic points needed to definitively clear or identify a suspect". Sacramento Bee.

Again, the emphasis squarely focuses on the July 31st 1969 envelopes and stamps, despite the much earlier Zodiac correspondences showing a tendency toward "few cells" being discovered. The July 31st 1969 letters were not available during the San Francisco Police Department DNA testing in the late 1990s, but why are the Kathleen Johns and Little List letters seemingly passing under the radar again in this latest round of testing, when the 2000 SFPD report clearly states that "cells were found" on these communications? The apparent omission of the Exorcist letter for submission this time round, despite being "loaded with saliva and cells" according to Criminalist Alan Keel, may further bolster the notion that law enforcement have no confidence in this being a Zodiac letter. Does the apparent reluctance to consider the Kathleen Johns and Little List letters for testing in 2018, suggest an equal lack of confidence?

Were the July 31st 1969 letters submitted in 2018 because current law enforcement do believe in the validity of the findings of the 2002 documentary? After all, Detective Poyser did state "Allen largely was cleared as a suspect in 2002, when investigators obtained a partial DNA sample from the envelopes that didn't match his. Poyser said that DNA profile only had a few markers on it, less than half of the genetic points needed to definitively clear or identify a suspect". Law enforcement must believe that DNA was found on the sealed side of the envelopes and stamps from the July 31st 1969 letters, hence why they are focusing on these communications again, in the hope of finding a full or more complete DNA profile using more advanced technology. If law enforcement really believed the DNA testing from 2002 was only achieved by swabbing the exterior of the July 31st 1969 letters, then their unreserved targeting of these communications would be misplaced. But they seemingly don't believe this to be the case. ​They apparently don't share the reservations of the Zodiac community. 

It could be regarded as contradictory that the majority of the Zodiac community believe the Exorcist letter to be a genuine Zodiac correspondence - one that is widely touted as the final confirmed communication by the Bay Area murderer by ardent sleuths and law enforcement alike, which according to Alan Keel is "loaded with saliva and cells" - yet was seemingly roundly ignored in 2002, and once again in 2018, when this letter by all accounts should be the first port of call regarding biological material. It strongly suggests that the exhibited confidence in the January 29th 1974 letter is totally misplaced - and the Zodiac Killer may never have returned in 1974 at all.

DNA AND THE DAVID TOSCHI CONTROVERSY

Mike Rodelli wrote in his book, 'The Hunt for Zodiac:The Inconceivable Double Life of a Notorious Serial Killer', that Alan Keel, Criminalist at the San Francisco Police Department, San Francisco, California from 1996 to 1999 "revealed that there were two letters in possession of the department that, in contrast to the "true" Zodiac letters, had abundant saliva and DNA-containing oral epithelial cells on them, that DNA was easily extracted from these two letters, and that the DNA extracted from these two letters matched between them. These two letters were considered forgeries, since the "true" Zodiac verifiable letters had not been licked by the sender." Mike Rodelli also stated "In contrast (to earlier communications), Keel analyzes two other letters, one of which is the 1978 forgery, and finds that this letter and one of the 1974 letters are loaded with saliva and cells. He then easily extracts DNA from both of these letters using the more primitive DNA technology of that time and finds that the DNA matches between those two letters, thus proving that one person sent both."  

The San Francisco Police Department DNA report clearly states that the 1978 letter was tested for DNA (which was obtained) and deemed 'not an authentic Zodiac letter'. The only other letter in 1974 processed for DNA (where cells were found), was the Exorcist letter. Therefore, the two letters described by Alan Keel "as matching between them" had to be the 1978 letter and the Exorcist letter. If the 1978 letter is 'not an authentic Zodiac letter', then the Exorcist letter is 'not an authentic Zodiac letter' either. But how can the San Francisco Police Department claim that the 1978 letter is not an authentic Zodiac communication, when you don't have the Zodiac Killer's DNA to test it against? The only two possible answers, are that (Inspector) David Toschi, accused of forging the 1978 letter, had his DNA compared to the 1978 letter and it matched, thus proving it couldn't have been written by Zodiac, or the DNA markers on the 1978 letter confirmed it was written by a woman. Females typically have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males typically have two different kinds of sex chromosomes (XY), and are called the heterogametic sex. 

If we conclude that David Toschi hadn't forged either the 1978 letter or the Exorcist letter, and the "DNA matched between them" as stated by Criminalist Alan Keel, then we are left to conclude that both were authored by a woman. However, if this had been determined to be the case, why not release this to the public and officially exonerate David Toschi beyond any doubt and clear this perceived stain on his character. This would be the least you would do for a respected and dedicated law enforcement officer, who served the San Francisco Police Department from 1952 to 1987. The fact they didn't do this, suggests that both the 1978 letter and Exorcist letter were not authored by a woman. But this once again places the spotlight back onto David Toschi as the only other option. 

It has been claimed that a palm print discovered on the Exorcist letter has ruled out Arthur Leigh Allen as the contributor. Michael Butterfield wrote "The San Francisco Police department also announced that Allen’s palm print did not match the palm print found on the Zodiac’s “Exorcist” letter of 1974". Zodiac Killer Facts. If this palm print ruled out Arthur Leigh Allen as the author of the Exorcist letter, then it could also be used to rule out David Toschi. If the Exorcist letter and 1978 letter are inextricably linked through DNA and therefore the stamps and envelopes were licked by the same individual, as inferred by Mike Rodelli (from the findings of Criminalist Alan Keel), then by logical extension, if David Toschi was ruled out as the author of the Exorcist letter by palm print, he would also be ruled out as the author of the 1978 letter. 

Deputy Police Chief Clement DeAmicis stated on July 14th 1978, regarding the Exorcist and 1978 letter "We are looking into the authentication of the letters. We have submitted a number of specimens of handwriting. Toschi's is one". Referring to other Zodiac correspondence he continued "We want to know whether they are authentic or whether they are written by somebody we know." Why wouldn't you just compare David Toschi's palm print to the Exorcist letter, as they had apparently done to rule out Arthur Leigh Allen. The San Francisco Police Department stated in 1999 "The Police Department has never made a statement regarding accusations that Toschi faked a Zodiac letter one way or another. We will confirm that not all Zodiac letters are authentic". This final line tells you everything you need to know. The only way they can claim definitively that "not all Zodiac letters are authentic", is if they have confirmed this with DNA. Efforts to harbor DNA from these communications began in the late 1990s - the same time this SFPD statement was released. This lack of transparency from the San Francisco Police Department, the bold claim of ruling out the 1978 letter through DNA (without having confirmed Zodiac DNA), the failure to openly rule out David Toschi using the palm print, and the reluctance to publicly exonerate David Toschi using the 'apparent DNA match between the 1978 letter and Exorcist letter', unfortunately tells you everything. If the San Francisco Police Department have ruled out the 1978 letter through DNA, and not because it's a woman, then it doesn't leave too many other options.

Zodiac researcher Mike Rodelli recalled a conversation with Alan Keel, Criminalist at the San Francisco Police Department, San Francisco, California from 1996 to 1999. He stated "In contrast (to earlier communications), Keel analyzes two other letters, one of which is the 1978 forgery, and finds that this letter and one of the 1974 letters are loaded with saliva and cells. He then easily extracts DNA from both of these letters using the more primitive DNA technology of that time and finds that the DNA matches between those two letters, thus proving that one person sent both." 

For these two communications to be matched definitively using DNA, "proving that one person sent both", they must have revealed full DNA profiles, otherwise, a partial DNA profile in one could not have proved conclusive. The DNA tests conducted by the San Francisco Police Department in the late 1990s, indicated that subsequent to 1974, only the January 29th 1974 'Exorcist' letter had been processed and "cells found". There was no mention of the other 1974 letters being processed. Therefore, one could conclude that the two letters discovered to match one another (according to Keel), were the 1974 'Exorcist' letter and the April 24th 1978 letter - and therefore written by the same individual.

The 1978 letter is listed in the report as "not authentic Zodiac letter." If the San Francisco Police Department concluded the 1978 letter was not an authentic Zodiac letter, then the 'Exorcist' letter would not be either. This brings forth some interesting questions.    

Michael Butterfield wrote an article entitled 'Zodiac DNA:The Magic Bullet' on his 'Zodiac Killer Facts' website, stating
"In the late 1990s, San Francisco police obtained DNA from a suspected Zodiac letter, and the DNA did not match Allen’s DNA".. 
The San Francisco Police Department surely wouldn't have used the DNA obtained from the April 24th 1978 letter for comparison, as they themselves had deemed it "not authentic Zodiac correspondence" and it had long been considered questionable material. Therefore, they must have obtained this DNA from another letter they "suspected" was a Zodiac letter, to compare to known suspects. But, if the 'Exorcist' letter (as Mike Rodelli suspects) and the 1978 letter matched in DNA, were written by the same individual, and by association were considered "not authentic Zodiac correspondence", then which "suspected Zodiac letter" did the San Francisco Police Department use to compare to Allen's DNA. 

In the San Francisco Police Department report, the only letters in which they stated "cells found", other than the 'Exorcist' letter, were the July 24th 1970 'Kathleen Johns' letter and July 26th 1970 'Little List' letter. Every correspondence tested prior to July 24th 1970 were labelled 'Envelope processed for DNA - few cells'. The 'Exorcist' letter came under scrutiny, alongside the 1978 letter, when Inspector David Toschi fell under the microscope in 1978 for forging one or both. So, would it be likely the 'Exorcist' or 1978 letter would be chosen by the San Francisco Police Department in the 1990s as a "suspected Zodiac letter" (and for the reasons given above), and used to rule out Arthur Leigh Allen's DNA? They surely would have turned to more credible sources. But they only had two other letters where "cells were found".

In 2002, Dr. Cydne Holt performed a new round of tests on some of the early 1969 Zodiac letters. Having failed to find suitable DNA on these letters (as suggested by the "few cells" findings of the 1990s), it has now come to light that the ABC Primetime documentary scoured the outside of the envelopes and stamps for DNA instead, thereby negating any findings as to the identity of the Zodiac Killer.

But why were they searching for Zodiac DNA, to compare to suspects such as Arthur Leigh Allen and Kjell Qvale, when: 
"in the late 1990s, San Francisco police obtained DNA from a suspected Zodiac letter, and the DNA did not match Allen’s DNA".
The new round of tests performed in 2002, would now suggest that the DNA they found in the 1990s from a "suspected Zodiac letter", was no longer believed to be from Zodiac. 

In 2018 "Vallejo police Detective Terry Poyser, who has worked the Zodiac case for four years, said his agency has submitted two envelopes that contained letters from the Zodiac Killer for a type of advanced DNA analysis that previously had not been available in the case. The department has three letters and two envelopes from the Zodiac, Poyser said. The envelopes each have a double stamp, which Poyser said was a trademark of the Zodiac. They originally were sent to the San Francisco Chronicle and the San Francisco Examiner, he said. The third letter was sent to the Vallejo Times-Herald, but does not have an envelope with it, he said." Sacramento Bee.

However, these are the July 31st 1969 envelopes and stamps already tested by Dr. Cydne Holt in 2002, which failed to produce any viable results. The hope is that more advanced DNA analysis may turn up something new. But what about the "suspected Zodiac letter" from the 1990s with "cells found." Couldn't this be examined using advanced DNA analysis? This letter has apparently already produced a DNA profile to rule out Allen, so why is it being ignored - or is it?

In a poll of 93 people conducted on this site, 85% believe the 'Exorcist' letter to be genuine Zodiac correspondence, whereas, in a poll of 94 people conducted on this site, only 54% believe the 1978 letter to be genuine Zodiac correspondence.
If Mike Rodelli's suspicions are true, then both the 'Exorcist' and 1978  letter are "not authentic Zodiac correspondence". If the San Francisco Police Department have long considered these two communications as not credible, then which "suspected Zodiac letter" was used to rule out Arthur Leigh Allen in the 1990s, and can it be used again for advanced DNA analysis?

We have long been led to believe the 'Exorcist' letter was the last confirmed communication by the Zodiac Killer. If the poll on this site were true of the entire Zodiac community, then 85% of people believe the 'Exorcist' letter to be genuine Zodiac correspondence. If the other 1974 letters were not tested or found to contain DNA (as the SFPD report suggests), then it is extremely likely a full DNA profile exists for the 'Exorcist' letter (shown at the top of this article).

For those who believe the January 29th 1974 'Exorcist' letter to be an authentic mailing from the Zodiac Killer with a full DNA profile, you should be asking "why hasn't this been run through GEDmatch."

I have just finished reading 'The Hunt for Zodiac: The Inconceivable Double Life of a Notorious Serial Killer' by Zodiac researcher Mike Rodelli. While I don't agree with everything in the book, it is an extremely well researched and informative addition to the Zodiac Killer story, even if you don't arrive at the same conclusions on the murderer's identity. For beginners and seasoned veterans alike, it will not disappoint as a comprehensive introduction to the case, as well as a complimentary addition to any existing knowledge you may already have. While fresh in my mind, I would like to explore one important feature of the book, regarding the topic of DNA, which is currently hanging over the case like a dose of skeptical optimism.. 

Mike tackles this problem in extensive detail, particularly with regards to the viability of the DNA tests conducted by Dr. Cydne Holt in the 2002 ABC Primetime documentary, in addition to his discussions with Alan Keel, Criminalist at the San Francisco Police Department, San Francisco, California from 1996 to 1999.

The disputed results of the 2002 'showpiece' are now well documented, however, Mike delves further into the DNA analysis with respect to two later 'Zodiac' letters, namely the 1974 'Exorcist' letter and the subsequent 1978 'I Am Back With You' letter, both mailed after a lengthy silence from the Zodiac Killer - leading some to question their authenticity. 

Mike recalls information of his conversation with Alan Keel: "In contrast (to earlier communications), Keel analyzes two other letters, one of which is the 1978 forgery, and finds that this letter and one of the 1974 letters are loaded with saliva and cells. He then easily extracts DNA from both of these letters using the more primitive DNA technology of that time and finds that the DNA matches between those two letters, thus proving that one person sent both." 

This is a stunning revelation (if correct) when we consider the findings of the San Francisco Police Department laboratory in the 1990s, in which there was a notable absence of saliva/DNA present on the envelopes prior to the 1974 'Exorcist' letter. Would a killer who seemingly didn't lick any of his envelopes or stamps throughout the majority of his letter-writing campaign, now suddenly break that habit in the 1974 and 1978 communications? The SFPD report, shown here, references the April 24th 1978 letter, claiming that the "DNA SAMPLE OBTAINED - NOT AUTHENTIC ZODIAC LETTER".   

This lack of saliva or DNA on the earlier letters may have become more evident, as Dr..Cydne Holt struggled to generate any tangible results from the sealed sections of stamps and envelopes using more advanced forensic DNA testing in 2002, compared to the earlier and more primitive testing undertaken in the 1990s. This apparent dichotomy brings into stark reality the essence of Mike Rodelli's argument.     

The bold statement of "DNA  sample obtained- not authentic Zodiac letter" opens up the proverbial can of worms. On first reading, it gives the impression that they have something to compare it to. How would they know the DNA sample obtained from the 1978 letter was not from Zodiac, if they didn't have Zodiac's DNA? If they had Zodiac's DNA in the 1990's, this wasn't the impression given by the search for Zodiac DNA in the 2002 ABC Primetime documentary.

"DNA  sample obtained- not authentic Zodiac letter" could imply they know the sex of the DNA present on the 1978 letter, and therefore by extension, not the Zodiac Killer (as suggested on many forums). But, it could imply they know the identity of the individual who contributed the DNA to the 1978 letter, and having identified that individual, know they were not the Zodiac Killer. This lack of clarity has led Zodiac researchers to question the transparency of the San Francisco Police Department.     

We know that a partial DNA profile from a letter can exclude suspects, but not definitively identify an individual. Therefore, the apparent (but unconfirmed) revelation of Alan Keel that "DNA matches between" one of the 1974 letters (possibly the 'Exorcist' letter) and 1978 'I Am Back With You' letter, would suggest that full DNA profiles have been obtained for both letters.
​If only a partial DNA profile had been obtained from the 1978 letter (much like the exclusion of suspects) it can only be excluded as matching a profile from the 'Exorcist' letter (if this had a full DNA profile), but not definitively matched to it.
As such, the statement of "DNA matches between those two letters, thus proving that one person sent both", would be indicative of a full DNA profile obtained from both the 1974 and 1978 letters in the 1990s.. 

For those who believe the 'Exorcist' letter to be genuine Zodiac correspondence, then this could imply that a full DNA profile may exist for this letter (or one of the 'lesser' 1974 letters), and consequently the profile of the Zodiac Killer. However, the search for Zodiac's DNA in 2002 negates this premise - unless the San Francisco Police Department knew all along that both the 'Exorcist'/74 letter and 1978 letter were not mailed by the Zodiac Killer. In fact, the Exorcist and April 24th 1978 letters were the only two communications mentioned with regards to the controversy surrounding Inspector David Toschi, who led the investigation for many years. He came under suspicion for forging the 1978 letter, after the San Francisco Chronicle columnist and author, Armistead Maupin, discovered that David Toschi had mailed fictitious fan mail to a series he had written, with the inclusion of the detective.  ​

​Armistead Maupin had first met David Toschi in 1976 while he was writing the 'Tales of the City' series. Wanting to energize the series with a murder mystery element, he contacted San Francisco Chronicle reporter Bob Popp, who then introduced him to David Toschi. The police inspector would make a debut appearance in 'Tales of the City' on September 10th that year, with a character based on his attributes. Further episodes would follow under his real name, as a friend and confidant to Armistead Maupin's fictitious Detective Inspector Henry Tandy. 

Here is an excerpt from the San Francisco Chronicle, published on July 14th 1978 "Police officials are investigating the possibility that Inspector David Toschi forged a Zodiac letter in The Chronicle in 1974 as well as a similar letter the newspaper received earlier this year. Toschi - who said he was interrogated officially last Friday about both letters - has denied writing any letters in the guise of Zodiac, the killer of six who eluded capture by Toschi for nearly nine years before the detective was transferred out of the homicide detail last week.
Deputy Clement DeAmicis, who had recommended the transfer to Chief Charles Gain, confirmed yesterday the probe was being extended".  

The fact that Inspector David Toschi was questioned regarding these two communications, only heightens the mystery surrounding Alan Keel's statement of "the DNA matches between those two letters, thus proving that one person sent both" and the San Francisco Police Department's report concerning the 1978 letter, in which it noted that a "DNA  sample was obtained, but indicated it was not an authentic Zodiac letter".

We don't know for certain that Alan Keel is referring to the 'Exorcist' letter when he stated "one of which is the 1978 forgery, and finds that this letter and one of the 1974 letters are loaded with saliva and cells".  However, since the 'Exorcist' letter for the large part has been widely regarded as the last confirmed correspondence mailed by the Zodiac Killer, as opposed to the other 1974 communications,  it would be the logical correspondence to test for Zodiac Killer DNA  when searching for the Bay Area murderer. Testing the other much more questionable 1974 mailings would not make as much sense - which is probably why the San Francisco Police Department report is blank or omitted next to these communications. 

The 'go to' mailing, when testing for Zodiac DNA from the 1974 letters, would undoubtedly be the 'Exorcist' letter rather than its dubious companions. So, if this was the case, and the statement of Alan Keel were true, then the 'Exorcist' mailing would not be an authentic Zodiac correspondence, and as a result, would quite possibly shrink the Zodiac reign by nearly three years.  

"Paul Avery and Kate Coleman who wrote the sizzling expose of Black Panther violence in the July 10th New Times 'have gone out of the area' after receiving the predictable threats," wrote Herb Caen. The bogus letter reopened old wounds, driving the populace to seek closure on the long-unsolved case. Zodiac tips quadrupled. "The Zodiac was shot to death by San Francisco police in March of 1976," a reader informed Caen, "about four months after he set the Gartland Hotel fire that killed thirteen people." Zodiac Unmasked. 
​
On July 19th 1978 a typewritten letter from Los Angeles arrived at the San Francisco Chronicle stating:
"I am the ZODIAC and I am in control of all things. I am going to tell you a secret. I like friction tape. I like to have it around in case I need to truss someone up in a hurry....I have my real name on a small metallic tape. You see, while you have it in your possession, I want you to know it belongs to me and you think I may have left it accidentally. I am athletic. It could be swim fins, or a piece of scuba gear. But maybe you play chess with me. I have several cheap sets in closets all over. I have my name on the bottom of the lid with the scotch tape....My tape is waiting for me all over California. Do you know me? I am the ZODIAC and I am in control."  

Sergeant John Lynch from the Vallejo Police Department interviewed Arthur Leigh Allen, with Allen stating that he traveled to Salt Point Ranch, located on the coast, due west of Napa on September 26th 1969 for a spot of scuba diving.
Clearly, Arthur Leigh Allen was a prime suspect for Zodiac, so it must have been obvious to Allen that he would incriminate himself by inserting "swim fins" and "scuba gear" into the narrative of the 1978 letter, unless you believe a double bluff. 
This letter followed the "I am back with you" correspondence mailed to the San Francisco Chronicle on April 24th 1978, just three months earlier.
Arthur Leigh  had been released from the Atascadero State Hospital on August 31st 1977, and this letter is often touted as his return to the persona of the Zodiac Killer after a lengthy  absence. Or was it more likely that somebody known or unknown to Allen was attempting to implicate him in the murders by dropping subtle clues pointing in his direction. 
The author of the July 1978 letter seemingly wants to push us in the direction of tape: "I like friction tape", "I have my real name on a small metallic tape," "I have my name on the bottom of the lid with the scotch tape," and "My tape is waiting for me all over California​." 
It appears like a selling job, trying to push the tape angle in connection with trussing people up, whereas the author may have had other intentions.

The Zodiac Killer mailed us a 'Bus Bomb' diagram on November 9th 1969 and April 20th 1970 employing the use of electrical circuits.
'Friction tape is a type of adhesive tape made from cloth impregnated with a rubber-based adhesive, mainly used to insulate splices in electric wires and cables. Because the adhesive is impregnated in the cloth, friction tape is sticky on both sides. The rubber-based adhesive makes it an electrical insulator and provides a degree of protection from liquids and corrosion. In the past, friction tape was widely used by electricians, but PVC electrical tape has replaced it in most applications today. The frictional properties of the tape come from the cloth material, which is usually made from cotton, while the fabric base protects electrical splices against punctures and abrasion. Wikipedia.' 
Was this the implication, with 'pipe bombs' having been discovered at Allen's residence, along with supposed bomb manuals.
But more credible, is the emphasis on "swim fins" and "scuba gear," considering Allen's penchant for scuba diving, and the opportunities for such pursuits provided by Lake Berryessa, Lake Tahoe and Fisherman's Wharf near to the Presidio Park- in three Zodiac linked crimes. 
Permanent marker pens, in conjunction with water resistant tape or labels are ideal for scuba divers and boat enthusiasts, for personalizing your equipment. dolphindiveathens.com/marking-your-dive-gear/. "I have my name on the bottom of the lid with the scotch tape....My tape is waiting for me all over California."
With Arthur Leigh Allen the prime Zodiac candidate for many years, isn't it plausible that somebody was attempting to throw Allen under the proverbial 'school bus' in an attempt to keep him in the frame, while keeping an interest in the Zodiac case burning bright. These two letters could actually be from Zodiac (depending on the validity of DNA evidence), and is simply a game by the real killer to keep the spotlight firmly focused on his convenient "fall guy."​

'I keep on hearing the police have caught me but they wont fix me just yet. I have laughed when they look so clever and talk about being on the right track'. Taunting words dragged from the 'Dear Boss' letter, presumably sent by Jack the Ripper and mailed on September 27th 1888, exactly 81 years prior to the Lake Berryessa attack on Bryan Hartnell and Cecelia Shepard. In this letter he taunts the police 'The next job I do I shall clip the ladys ears off and send to the police officers just for jolly wouldn't you. Keep this letter back till I do a bit more work, then give it out straight. My knife's so nice and sharp I want to get to work right away if I get a chance. Good Luck. Yours truly. Jack the Ripper'.
Bearing in mind the wording on the Ripper letter, the 'next job' the Zodiac Killer undertook after the Lake Berryessa attack was the murder of Paul Stine, in the wealthy district of Presidio Heights on October 11th 1969. Despite the callousness of the crime, the Zodiac Killer did refrain from the unthinkable, although he did remove an item from the crime scene, tearing off a section of Paul Stine's shirt and sending it to authorities. Two days after the murder, this letter, not dissimilar to the 'Dear Boss' letter, arrived at the San Francisco Chronicle on October 13th 1969. It said 'The S.F. Police could have caught me last night if they had searched the park properly instead of holding road races with their motorcicles seeing who could make the most noise. The car drivers should have just parked their cars and sat there quietly waiting for me to come out of cover'.
He then backed this up in similar fashion one month later with the 'Bus Bomb' letter on November 9th 1969, stating 'I gave the cops som bussy work to do to keep them happy. I enjoy needling the blue pigs. Hey blue pig I was in the park -- you were useing fire trucks to mask the sound of your cruzeing prowl cars'.

It again emphasized that the Zodiac Killer was 'still here', almost reminding us of the fact he had never gone away. In many peoples eyes this aroused suspicions, that this letter was a poor imitation, crafted from the inspiration of previous correspondence. Nevertheless, this letter also signed off with the words 'yours truly' once again, in extremely similar fashion to the 1978 letter. See here. 
In the 1978 letter the author stated  'I am waiting for a good movie about me' and in the 1987 letter the author stated 'Cars make nice weapons. The pigs can catch me if they can find me out there. Just like in the movie The Car'.
The Zodiac Killer, if we take these two letters as genuine, appeared to be reminding the American public that he was definitely far from dead, as some press speculation had considered, due mainly to the Zodiac Killer's lack of correspondence subsequent to the 'Exorcist' letter in early 1974. But now he was back..... in fact he had never gone away.
Yours Truly-Jack the Ripper was a short story by Robert Bloch, published in Weird Tales in July 1943 was made for television on 11th April 1961, directed by Ray Milland and starring Boris Karloff, John Williams and Beverly Powers. See here.
In the story Sir Guy (John Williams) attempts to convince the police that Jack the Ripper is far from dead- in fact he is very much alive and still claiming victims- this time in New York. (see Albany Letter). In one line he says 'Now none of these Ripper murders received the notoriety of those in London, many went almost unnoticed, but they did occur' and went on to elaborate 'I've studied everything about them, especially the dates and the pattern made by those dates, I've studied their rhythm, the solar and lunar rhythm'. See here.
The police thought these new murders were a copycat, attempting to replicate the murders of Jack the Ripper, and poured scorn on the idea he was still around. Could the Zodiac Killer have read the book or viewed this television episode and deliberately signed off these later letters with the phrase 'yours truly' to again ridicule the police and authorities, who thought, that he was dead also.