I found this registration card just a few days ago. It shows that Harvey was in the vicinity where Donna Lass, a victim of the Zodiac Killer from September 6, 1970 was last seen.
As I searched the area for other cold cases, I found some that may have a connection to the Zodiac Killer case as well.
There are keywords that I look for in the cold case files that trigger my suspicion that they are involved. The words I look for are ones I know meant something to Harvey or any of the other unsolved cases that belong to the Zodiac.