The case of the Zodiac Killer is riddle wrapped in a mystery inside an enigma, to borrow a phrase from Winston Churchill. But's it's more than that. Not only do we not know the answer to the ultimate question at the heart of the long-enduring mystery - i.e. who was the killer - but, if we're being honest with ourselves, we have to admit that we know distressingly little about some fundamental elements of the criminal and his crimes.
What was he killer's motivation? To what extent was the man honest in the letters he wrote? Why did he choose to act in such bizarre fashion at Lake Berryessa? Was he also responsible for the death of Cheri Jo Bates? What about Robert Domingos and Linda Edwards? Did he ever really intend to carry out any of his bizarre threats against school children? Did he abduct Kathleen Johns? Does the 340 conceal a legitimate message? What about the other two unsolved ciphers? Did he really just stop killing or did circumstance somehow intervene and force him to discontinue his criminal exploits?
However, what is remarkable about the case is not just how much we don't know, but how much we don't know in light of how much we do know. I like to describe the situation by saying, much is known about the case, but precious little is truly understood.
The various investigations into the Zodiac Killer embody a remarkable amount of physical evidence: fingerprints, palm prints, shell casings, multiple eyewitness accounts including multiple people who conversed with the suspect, tire tracks, a mitochondrial DNA profile (Cheri Jo Bates's attacker), a partial nuclear DNA profile, handwriting, etc. The list goes on and on, culminating with the twenty-plus pages of direct psychological evidence penned by the hand of the killer himself.
Many a difficult murder case has been solved with considerably less evidence. Furthermore, with other serial criminals, a large collection of evidence usually paints a clear picture of the perpetrator. The various pieces are mutually supportive and when taken as a whole they fit together to form a consistent, meaningful image, like pieces of a puzzle.
The case of the Zodiac, on the other hand, has a completely different dynamic. Relatively little of the evidence is mutually supportive. Incrementally new pieces of evidence often not only fail to answer outstanding questions, they instead raise new questions of their own; or worse yet, they contradict previously accepted answers. Moreover, with the investigation being stagnant for so long, the passage of time has not served to solidify existing beliefs, but rather germinated seeds of doubt and hence increased the level of uncertainty.
Instead of the evidence being analogous to a typical puzzle where each piece is uniquely shaped and correctly interlocking the pieces conveys an unambiguous, colorful image, the evidence in the case of the Zodiac is analogous to a puzzle of a different sort. In this puzzle, all of the pieces are squares such that any piece can fit together with any other piece. Further, instead of an unambiguous colorful image which constrains the way the pieces are to be connected, the image is constructed from simple grayscale blobs which provide an additional level of ambiguity.
In the end, the way a person puts the pieces together often depends more on what he or she is attempting to prove or disprove rather than the message that the puzzle originally conveyed. In this sense, the evidence is functioning like an elaborate Zodiac Killer Rorschach test. With a Rorschach test (also known as an ink-blot test), what a person sees in the ink blots has more to do with the inner workings of the observer's psyche than it has to do with any real representation of an image. The situation with the Zodiac Killer evidence is similar. Even though the evidence clearly originated from circumstances that represented objective truth, the way a person puts all the ambiguous pieces together often depends, primarily, on what he or she wants to find in the evidence.
Some people want the personal satisfaction of believing they are the only one who can understand a mad genius, and so they find a mad genius. Others are comforted by the notion that the killer was an incompetent fool, so they find a man who undoubtedly would have been caught had it not been for an incredible run of luck. Of course, the most common instance of this phenomenon happens when a person who has a "suspect" finds that suspect in all aspects of the evidence.
This situation also reminds me of a line from Carl Sagan's 1985 novel Contact. In the story, the people of Earth receive a message from intelligent life elsewhere in the universe. The various different groups on Earth interpret the message in different ways, largely based on pre-existing beliefs. Sagan wrote:
The message was a kind of mirror in which each person saw his or her own beliefs challenged or confirmed.
In the case of the Zodiac, few people view the ambiguity in the evidence as a challenge to their beliefs, so that part is not really applicable. But otherwise, the quote is interestingly analogous to the serial-killer saga.
So, the question is: what do you see in the case evidence? How much of it is objective truth, and how much is a reflection of your own beliefs?