My family believe it was Richard Gaikowski, who was briefly looked at as a suspect in 1986, but it never went any further. This was obviously not in the public domain at the time.
However, there is a website dedicated to solving the Zodiac Killer case (zodiackiller.com), and they were contacted by an informant in 2008 who gave them Gaikowski's name. As a result, the website started looking into him, and received lots of information from members of the public. The information they received including photographs of him from the time of the killings, samples of his handwriting and audio files of him speaking.
The first thing that's obvious, from looking from photos, is just how much the artist impressions of Zodiac (taken from witness statements) resemble Gaikowski.
The website arranged a meeting with Nancy Slover, the police dispatcher who spoke to Zodiac after the Blue Rock Springs attack, and she identified Gaikowski's voice as the one she heard in July 1969.
Carol Stine, the wife of victim Paul Stine, recognized Gaikowski as having attended her husband's funeral.
Furthermore when the Department of Justice Questioned Documents division looked at Gaikowski's handwriting, back in 1986, they found strong consistencies with Zodiac's writing. This alone though wasn't enough to make him a prime suspect.
To those in my family however, Gaikowski was a suspect even BEFORE all this, and in fact it was a member of my family who first urged police in Napa County to look into him.
The victim my grandmother was related to, once moved from CA to Albany, NY, and Gaikowski suddenly upped sticks from CA and moved to Albany just a few weeks later. While in Albany, he worked in the same building as the victim's then husband. Not long after she and her husband moved back to CA, Gaikowski then also relocated back to CA.
Now his name is in the public domain, and has been for many years, it's given my family the opportunity to talk openly about their long-held belief that Gaikowski was Zodiac.