I mention this as a cautionary warning in regards to the perceived gaps in Zodiac Killer activity, such as the widely touted 4-year hiatus in Zodiac communications between 1974 and 1978. It's one thing believing a letter hasn't met the threshold of evidence to declare its authenticity, but quite another claiming it's a hoax based on nothing more than looking at it. With that said, I will continue the following observations based upon the April 24th 1978 and May 2nd 1978 letters being genuine Zodiac material, because the evidence against is practically zero compared to the evidence for.
The Hillside Strangler murders, eventually discovered to be both Kenneth Bianci and Angelo Buono Jr., took place in Los Angeles between October 1977 and February 1978, resulting in the murder of ten women and girls who were abducted, raped and tortured. The Hillside Strangler murders took the police far afield, with their investigation spreading to New York and the United Kingdom in pursuit of the killer/s identity, unsure of whether more than one individual was involved in the horrific crimes. One person in 1978 would have had a keen interest in the unfolding newspaper coverage, which makes a multiple page letter mailed to police in respect to this case in January 1978, very interesting. On December 7th 1969, the Zodiac Killer mailed a letter from Fairfield threatening to kill a cop, which after he would turn himself in. On January 19th 1978, Los Angeles Chief Daryl Gates held a news conference on television and stated "The fact we have not had any more murders in the last five weeks suggests that the individual or individuals are not operating. Hopefully that will continue to be the situation. I'm hopeful the killer or killers will come forward and talk with us and give themselves up". Daryl Gates informed reporters that the manhunt was the biggest since the Tate-LaBianca murders which cost 500, 000 dollars.
The Zodiac Killer would apparently reappear on March 13th 1978 and April 24th 1978 stating "This is the Zodiac. Tell the press that I am back in San Francisco" and "I am back with you", before mailing another letter from Los Angeles threatening to kill five people, in which Chief Daryl Gates was number one on that list, with former Los Angeles Chief Ed Davis second. Susan Atkins, part of the Manson family responsible for the Tate-LaBianca murders, was also mentioned fifth on the list mailed to the KHJ-TV studios. As such, we have a letter postmarked January 19th 1978 promising proof to the claims of murder and offering to give himself (and "a friend") up to police, in response to a news conference hosted by Chief Daryl Gates, who asked the killer or killers to give themselves up - followed by another letter mailed from Los Angeles on May 2nd 1978 threatening Chief Daryl Gates, who spearheaded the Hillside Strangler investigation. Who knows whether this was the Zodiac Killer interjecting himself into another major investigation, but it's certainly a possibility.