In respect to the Zodiac timeline, it was sent 15 months after the disappearance of Donna Lass from Stateline, Nevada on September 6th 1970.
The communication claims to have evidence that with proper investigation would disclose the name and identity of the infamous 'Zodiac Killer of California.'
The author of this letter would receive a reply dated December 23rd 1971, informing him that the matter was not within the investigative jurisdiction of the FBI and he should advise the San Francisco, California Police Department of any information which pertains to the Zodiac Killer. Signed by Director, John Edgar Hoover.
The identity or signature of this individual is redacted, with a return address of some description- therefore determining its sincerity or otherwise is difficult to determine. One would like to think this was an extremely concerned citizen or somebody from an official agency, frustrated enough to mail a letter to the FBI offices (handwritten letter below).
Senator George Stanley McGovern was a U.S. Representative and U.S. Senator born in Avon, South Dakota, approximately 100 miles from Sioux Falls, South Dakota, which may reflect his interest in the case.
Another communication in the form of a Christmas card was mailed to the sister of Donna Lass, Mary Pilker, in Sioux Falls, South Dakota on December 27th 1974. The Christmas card read 'Holiday Greetings and Best Wishes for a Happy New Year', followed by the handwriting "Best Wishes, St Donna & Guardian of the Pines."
It is unknown whether this card had sinister connotations in respect to where Donna Lass may be buried (somewhere in between the pines), or whether it is just misguided support. There are, however, some similarities in the handwriting of the John Edgar Hoover letter and the Christmas card mailed to Mary Pilker (shown below).
Handwriting comparisons in the Zodiac case are the topic of much debate, as they are extremely subjective, and should always be used in conjunction with other evidence.