By Francesco Amicone
Updated on December 18, 2020
“Foothill Blvd Hillcrest, 16 mi fro 570°” is the plaintext of the encrypted note contained in a letter from Zodiac postmarked June 26, 1969. The serial killer claimed to have reported in the cipher where he had placed a bomb.
The decryption indicates that the target zone was a specific area of San Leandro where the Alameda County Sheriff’s Station in San Leandro “Eden Township” (Google Maps) was located.
This is the third correction of the decryption of the Mt. Diablo cipher published on this blog in 2018. The currently version which contained the words “mi far” (miles far) instead of “miles” was shared on Reddit a few months ago. On December 18, 2020, I updated the “mi far” version switching “far” with the British “fro”.
Two replacements of this decryption are in each of the other Zodiac’s ciphers. You find all of them here.
The substitution letters/numbers is correct (table at the bottom of the page). I think Zodiac added a full turn to the degrees (210° + 360°) to mock who would have attempted to decrypt his note.
Location of the bomb
The bomb was placed on Foothill Blvd at Hillcrest, in San Leandro, about 16 miles from the peak of Mt. Diablo, apparently, in the surrounding area of the Alameda County Sheriff’s station “Eden Township” .
In February 1970, 12 Alameda County deputy Sheriffs were charged for having violated civil rights law (here’s the source; The Napa Register, February 3 and March 24, 1970).
“Mi fro” or “miles”?
The FBI analysts in 1970 supposed the presence of the word “miles” in the text (FBI Vault – Zodiac, part 5, pp. 167 – 168; FBI laboratory worksheet on the Z32). Zodiac could have used the abbreviation “mi” followed by the dialectal British word “fro”. However, it cannot be excluded that he may have done an enciphering mistake by switching the substitution for the L of “miles” with that for “F”. In this case, there would be the word “miles” in place of “mi fro”.
Map and cipher together
Zodiac wrote “the map coupled with this code will tell you where the bomb is set” (Zodiac’s letter and map – June 26, 1970), implying that the cipher and indication on the map were complementary, and the solution was based on both. One month later, Zodiac suggested the code was related to inches and radians (post scriptum, “Mikado letter”, FBI report), which it might mean he had enciphered an angle and a distance.
Set Zodiac’s compass indicator to the magnetic north of 1970 in the San Francisco Bay Area, 17° east of north (Historical Magnetic Declination, NOAA; the reference). Please, spin it clockwise 570° (a full turn and 210°). Unite the four line segment of the quadrants along the direction of the indicator with the help of a ruler. The arrow will point the target zone.
The angle and measurements could be verified with this image.
You will note that the arrow of Zodiac’s compass is not perfectly aligned with the vertical axis of the map but faces 2 degrees farther to the east with the respect of the true north. Be sure to make the measurement of the angle starting exactly from the arrow head or your indicator will point an adjacent area to the target zone.
A video explanation is here (select HD)Replacements
The Mt. Diablo cipher is composed of 29 different symbols for 15 letters, 5 numbers and the symbol of the degrees. The replacements numbers/letters follow the descending alphabetical order: 0 → W to 9 → N.