Hartnell's Car Door, the Zodiac Killer left a messege right after the attack with the previous dates of the other murders he committed.

2nd Letter sent to By Press. The zodiac explained what he plans to do in his spare time
3rd Letter sent to Bay Press by the Zodiac Killer.
A poem the Zodiac Killer sent to the bay press, He is telling the press why he does the thigs he does.

4th Letter sent to the Bay Press. The zodiac killer admits he murdered the taxi driver, and he warns he is likely to target school kids for his next victims
One of the last letters sent to Bay Press. Up to this day there are symbols that are yet to be decipher in the message of the Zodiac Killer

The Zodiack Killer alphabet. He used all these codes to inscribe messages in the letter sent to the Bay Press