Last month I summarized my analyses of Z13, the dots of the VMF (earlier I called it ZF) symbol, and the “sorry no cipher” note, combined, as they turned out to be interconnected. Here, I summarize my analyses of the Z32 cipher, the Halloween communication (also including the envelope, symbol and note), and the Sierra postcard, as it turns out that these communications are not only related to each other but, in fact, focus on the very same message: a bomb on Route 84 near Vallejo Mill and Paradise drive, Fremont. (You can derive this location yourself in five minutes, from the postcard, with no need to solve any cipher or interpret any puzzle, as I explain below.) When puzzles are thus interconnected, one solution (of Z340 in this case) often provides an initial insight (here: Zodiac chose homophonic ciphers with fixed transpositions plus irregularities) that paves the way to resolving all related puzzles. Subsequently, it is worthwhile to zoom out and examine the emerging, coherent picture. Thus, for example, we see that the same message was given first in cipher (April 1970), later in convoluted but redundant puzzles (October 1970), and finally in plain sight (March 1971). This escalation demonstrates an increasing pressure from Zodiac that we find the bomb; had he not suddenly vanished for several years after sending the postcard, who knows what he would have done.

The Mount Diablo map was provided in conjunction with Z32, but turns out to be identically related to the Sierra postcard. The Z32 solution and the postcard crosshairs symbols contain the very same message: east 3.0 rad(ians) 3.3 (inch), leading from Mount Diablo to Paradise drive, Fremont (with 6.4 miles/inch and a 16 degree offset to account for magnetic declination, as specified on the map). The message is most evident in the postcard, where these are simply the angle + distance separating the two crosshairs symbols, as you can easily verify yourself in a few minutes (rescale the stamp and postcard to standard, 5 by 7 inch postcard dimensions). In Z32, the message is in a cipher: heavily homophonic, diagonally transposed, somewhat in depth, and of self-supporting location and cryptanalytic attributes. Given the Z340 insight and the clues Zodiac provided, Z32 is not so difficult to solve, as I explained here (in retrospect, I wish I started with the more straightforward postcard rather than vice versa…). The agreement between Z32 and the postcard confirms and narrows down the bomb location, and corroborates both the postcard interpretation and the cipher solution. In particular, the repeated message substantiates the diagonal Z32 transpositions, also confirming that the dots in the VMF symbol indeed encode the transposition rules of Z340, Z13, and Z32, compactly and cleverly.

The Halloween communication had four parts (envelope, front, inside, and back), each of which is shown to convey the same Vall+ejo+Mill message, independently. In each of the four parts, “Val” was provided as a letter game, “ejo” as a reference to an eye (“ojo” in Spanish, familiar to many in 1970 California) with an O->E indication, and “Mill” as either a symbolic reference or just the letters M+il+L. Zodiac knew that the interpretation of each element was ambiguous, and so provided more than sufficient redundancy. In addition, he offered clues for (Route) 84, PARADiCE (drive), BOOM, etc. The VMF symbol probably gives the initials of Vallejo+Mill+Fremont (so, unfortunately, still no clue to Zodiac’s identity), and there is a low-likelihood reference to the Fremont seal. Additional Fremont references may possibly be found in the Sierra postcard, which deserves further study. In retrospect, we could have solved the Halloween communication independently, as the Vallejo Mill bomb is the only simple theory that directly explains all previously mysterious Halloween clues, and even exposes overlooked hints. For example, did you notice that among the many letters Zodiac sent to the S.F. Chronicle, only the Halloween envelope included the second address line, “5th – MiSSiON”? Like every other Halloween element, this addition had a deliberate purpose – do you see it? A few such clues, some of them quite clever, are pointed out here without much detail, as it might be more convincing if you find them yourself.

So, did Zodiac actually plant and arm the bomb on Route 84, as he claimed? Most probably, yes. The repeated and increasingly transparent communications demonstrate a mounting pressure from Zodiac to identify the location, so something was hidden there. Zodiac discussed his bomb plans at length, detailing over time its target, components, first design, testing, and storage. He uncharacteristically and apparently honestly complained about the labor of digging for such a massive bomb and adjusting its trigger mechanism, and admitted the failure of his first model. Zodiac then provided his improved, second design, including a more sensible trigger mechanism (e.g., the timer is needed to confirm a long vehicle before detonation), and made demands before arming it. After his demands were not met, Zodiac indicated that the bomb was already armed and explained the delay in its trigerring. Later, he went to the trouble of preparing Z32 as a clue for finding and disarming the bomb before a school bus triggers it in the fall of 1970, and sent a subsequent clue (rad+#inches) to facilitate the Z32 solution. He didn’t give up at this point: months later, he provided the impressively thought-out network of interconnected Halloween puzzles, and before vanishing, sent the Sierra postcard after the bomb was already due to explode. Such deliberate, ongoing, demanding, focused, and even retroactive efforts indicate at a high probability that the bomb was actually planted and armed, but luckily failed.

The postcard narrows down the search box, now limited only by paper-measurement precision (ours and Zodiac’s) rather than by the two-digit precision of Z32. The repeated “Vallejo Mill” clues focus attention on the southwestern part of the Route 84 section entering Fremont, where the elevated wayside mentioned in the bomb design flattens out. This location precisely aligns with Paradise drive, rendering the exact spot even more probable. And at this very location, two conspicuous objects appear to protrude among the trees above the wayside. The objects present as related to each other, with evidence for circular apertures aimed above the road; they match the photoelectric switches in Zodiac’s design, in terms of position, height, orientation, and size. In summary, there is a high probability of finding a failed bomb in this section of the wayside, probably connected to the two protruding objects. Such a device should be very carefully removed, and its inner parts examined forensically: Zodiac presents as highly intelligent (albeit a sloppy cryptography amateur), but he could not foresee the technological advances in forensic science.

Edit: A further study of the Sierra postcard revealed a thinly concealed, shifted-alphabet, cipher. The message, "Fremont NNE", with almost no ambiguity as this is a fixed-key (not a homophonic) cipher, cannot be coincidental. This message is yet another redundant confirmation of the bomb location, and indicates that the postcard focused mainly, albeit perhaps not exclusively, on the bomb.