The FBI file read: 'Re Butte tel to Bureau 11/21/69.
Enclosed herewith for the FBI laboratory are two copies of the letter received by (redacted) San Jose PD, San Jose, California on this date.
As noted, the message reads (redacted).
The laboratory is requested to have the Cryptogram Section attempt to immediately decipher the message and furnish the results to the San Francisco Office.
For the information of the Bureau and Sacramento, after a cursory analysis, officers of the San Jose, California PD, believe widow in message may be Mrs (redacted), San Jose, California. San Jose PD instituting 24 hour surveillance on Mrs (redacted).'
A further page read:
11/24/69 - 'Copy of handprinted note beginning "There's no doubt I will do my Thing!" and ending with the symbol (drawn crosshairs). Results of examination:
The ciphertext portion of Qc33 is not readable using previously recovered key. Its brevity precludes further cryptanalytic examination at this time. The furnished interpretation of "- + 62 + -" as representing "1620" can be neither proved nor disproved analytically. The = between "November" and "8" could represent the number "2". Such interpretation would be based on inferences drawn from the date of the postmark, not from cryptanalytic examination.' (see below for FBI files).
However, there may be a much simpler explanation, bearing in mind the Zodiac Killer's last two communications on November 8th and 9th of 1969 - in particular the Dripping Pen card.
Bearing in mind the Bus Bomb letter mailed on November 9th 1969 had already claimed 7 victims, then had an 8th victim already been claimed (presumably a male victim, if the mention of a widow was relevant). We would be looking for a victim between November 9th and November 21st. A future victim in November would likely have been murdered between November 21st and November 30th.
The phrase "There's no doubt I will do my Thing!" seemed to suggest an impending threat - hence the urgency of 24-hour surveillance on the woman. Therefore, the use of the term "widow" in the FBI file may likely not carry a sinister connotation of a prior murder.
The Zodiac Killer was certainly looking for a ninth and tenth victim on December 20th 1969 when he mailed a further correspondence to Melvin Belli's residence.