The man who wrote the letters was the killer.
Several times he gave info only the killer would know, and in two letters he enclosed a bit of the bloody shirt of Cab driver and Zodiac victim Paul Stine.
In my 2008 post which was quoted recently, I stated that I thought that there may have been more than one killer. It IS possible, as KarlK suggests that the writer of all the letters might not have killed at all.
However, the big stumbling block as you point out, is that bit of bloody shirt. IF the writer did not kill anyone at all, he had to have obtained the shirt piece from the person who did kill Paul Stine.
It is true that the writer, who named himself the Zodiac, did come forward with some facts about early cases which were not known to the general public. His earliest letters could be matched to later ones by the handwriting, excessive postage, and by the inclusion of pieces of Paul Stine's shirt.
That said, Zodiac the writer made some false claims and promises in his letters which show him to be a liar. This is most noticible in his "score" tallies. For example:
- On 9 November 1969, Zodiac claims to have killed 7 people in the previous December, July, AUGUST, September, and October. This was the beginning of his score keeping. In FACT, the only two unsolved homicides for August were Kathy Snoozy and Deborah Furlong. Those murders were later solved and another man was convicted of committing them.
- On 27 December 1969, He claims an eighth murder. There were two unsolved cases at the time, but Zodiac gives no details whatsoever.
- 22 March 1970, Kathleen Johns and her baby are abducted by a man whom she believes resembles an artist sketch of Zodiac. There is NO mention of the incident by Zodiac for months. He writes four letters before he bothers to mention and take credit for it. Everything he states about it has already been reported in the papers - nothing added by Z.
- 19 April 1970, He claims a total of 10. Again no details at all.
- 26 June 1970, He now claims a total of 12 killed, and indicates that he has killed a man sitting in a car with a .38 pistol. In fact, a police officer had been shot in such a manner, but by the time the letter arrived, police had already issued an arrest warrant for the man who actually did it.
Zodiac indicated on two occasions that a bomb would be set off and it never occurred.
24 July 1970, He claims 13 killed, and again on 5 October 1970, it is still 13.
BUT in the meanwhile, on 26 September 1970, Donna Lass goes missing from Lake Tahoe. Months later, he hints that he is responsible in his "Peek through the Pines" card of 22 March 1971.
When Reporter Paul Avery makes a connection between the Zodiac and the murder of Cherri Jo Bates in Riverside, Ca back in 1966, Zodiac takes credit for it and says that there are many more. He ups his "score" to 17 plus. Again no details what so ever.
My point is that Zodiac the writer liked to play games and he lied a lot. SO what can you believe in ANY of his writings? He obviously took credit for crimes that he did not do, and he did not claim others that Graysmith suspects him of doing.
There are a number of still unsolved murders in California which are related to each other. BUT whether or not they were committed by the same guy who killed Paul Stine is hard to say.
Again, it is my feeling that there were more than one serial killer active in California back then and many of the possible Zodiac Victims may have been murdered by others.
Zodiac gave himself a lot of press and built himself a legend. I think that he probably did kill people, but I think that he was much more into writing and game playing. There are a lot of unsolved cases and a lot of loose ends.