Prime suspects in Kintry and Dyer murders
Kinderman states that only Georgetown homicide and those with access to their files knew the truth about the Gemini killer's gimmick of cutting off the index finger and making the zodiac sign on the palm. Everyone else was fed misinformation about the wrong finger and the sign on the back.
Since investigators aren't generally in the business of seriously considering supernatural explanations for murder, Georgetown PD's initial prime suspects for a copy-cat killer would have to be someone within the police department who had access to the files. In both the novel and the film, having someone within Georgetown PD as a red herring prime suspect would have made for an interesting side plot.