All of the letters have been cracked except for one. The others were cracked by a couple who gave the FBI a heads up assistance-wise.
The above has been mentioned in several books already...
The Zodiac originator of the letters was in all likelihood:
1. Ex-Navy
2. Cryptographer and Radio Man in the Navy.
Even when revealing himself - it’ll turn out that all he is doing is simply revealing all - all being all Navy Cryptographers and Radio Men of the Navy that came shortly before, during and after his term of service.
That’s a long and indistinguishable list...thanks to DOD Navy training and regulations that makes such personnel non-unique location-wise for security purposes.
=8-)