So who wrote the letters??

So here's a question I had that led me to a theory... if Bob Vaughn (the guy Gyllenhaal went to visit that creeped him out) said:

"Rick (Marshall) didn't draw any posters. Mr. Graysmith, I do the posters myself. That's my handwriting."

Then who wrote the letters??
After talking with a colleague of mine, that became the only question I though went unanswered in the film.

Trying to connect the people in the third act of the film, this is a theory I came up with. And sure, it's probably a shot in the dark...

Theory:
The Zodiac is actually three men working together.
Bob Vaughn wrote letters.
Rick Marshall created the codes.
Arthur Leigh Allen committed the murders.


If this has been asked or answered already, my apologies. Thanks for any insight!