On Tuesday, April 21st 1970, the San Francisco Chronicle ran an article entitled The Bloody Satan Murder stating "An autopsy yesterday morning showed Salem was slain sometime last Wednesday", which was April 15th 1970. The introduction to the article highlighted the murder of Robert Michael Salem in his 745 Stevenson Street, San Francisco apartment, describing the "macabre mutilation of the renowned lighting designer" found Sunday evening (April 19th). On Monday, April 20th 1970, the San Francisco Chronicle produced their first article on the murder, stating "A bizarre murder - with the words Satan Saves Zodiac written in blood on the expensively decorated hippie-style pad - was discovered last night at 745 Stevenson Street". The pseudonym "Zodiac" and the drawing of a religious Ankh was also found scrawled in blood on the apartment wall. It is clear, that despite Robert Salem being murdered on April 15th 1970 (or thereabouts), his murder was not discovered until Sunday, April 19th 1970. The story not released publicly until it appeared the following morning in the San Francisco Chronicle newspaper.
If the Zodiac Killer was not the murderer of Robert Salem - and didn't have inside knowledge of the discovery on April 19th - the first time he would have become aware of his pseudonym being found at a murder scene was when he bought his morning newspaper on April 20th 1970. The very same day, a letter arrived at the San Francisco Chronicle stating "PS I hope you have fun trying to fiygure out who I killed" and "I have killed ten people to date". After four months of not writing, was the timing of this letter really a coincidence - falling on the same day his bloody pseudonym was reported on the wall of a murder victim. The Zodiac Killer could easily have just written the victim count once, next to his crosshairs, but specifically added "I have killed ten people to date", suggesting that we should include the month of April into the equation. The Zodiac Killer was seemingly attempting to push us into concluding he was the murderer of Robert Michael Salem. By including a short code of thirteen characters, which included the three number eights (Christ the Redeemer in Christian numerology, and the opposing value of 666, the number of the beast), was the phrase of "I hope you have fun trying to fiygure out who I killed" trying to lead us to "Satan Saves" on the wall of the apartment?
If the Zodiac Killer was not the murderer of Robert Salem - and didn't have inside knowledge of the discovery on April 19th - the first time he would have become aware of his pseudonym being found at a murder scene was when he bought his morning newspaper on April 20th 1970. The very same day, a letter arrived at the San Francisco Chronicle stating "PS I hope you have fun trying to fiygure out who I killed" and "I have killed ten people to date". After four months of not writing, was the timing of this letter really a coincidence - falling on the same day his bloody pseudonym was reported on the wall of a murder victim. The Zodiac Killer could easily have just written the victim count once, next to his crosshairs, but specifically added "I have killed ten people to date", suggesting that we should include the month of April into the equation. The Zodiac Killer was seemingly attempting to push us into concluding he was the murderer of Robert Michael Salem. By including a short code of thirteen characters, which included the three number eights (Christ the Redeemer in Christian numerology, and the opposing value of 666, the number of the beast), was the phrase of "I hope you have fun trying to fiygure out who I killed" trying to lead us to "Satan Saves" on the wall of the apartment?
What is crucial here, is the timing of this Zodiac communication in relation to the newspaper article reporting the murder of Robert Salem. If the letter was insinuating he was responsible for the 745 Stevenson Street murder, then after reading the San Francisco Chronicle on Monday, April 20th 1970, the Zodiac Killer would have had to access (or buy) his writing paper, envelopes and stamps - create a thirteen character code, recall the Officer Brian McDonnell murder on February 16th 1970, compose the letter and draw his "string of bombs" diagram almost immediately - before mailing the letter in time for it still to be collected, processed and postmarked AM on the morning of Monday, April 20th 1970 (before noon),. Is that a reasonable proposition? If this were the case, it could imply that the 13-Symbol cipher was created with no forethought, was contemporary in nature and possibly related to the murder of Robert Salem (hence the numbers).
The alternative could be that the April 20th 1970 letter was created in advance of the San Francisco Chronicle article reporting the murder of Robert Salem. This could mean that the Zodiac Killer knew of the murder prior to the details being released in the newspaper, because he was the murderer of Robert Salem at 745 Stevenson Street. It just depends on what credence you place on the timing of this letter and the emphasis you place on "I have killed ten people to date" to mean "I have killed ten people to April 20th 1970", thereby highlighting the importance of April to the victim count.