I rewatched the part about the retired cop they interviewed. He was shot and killed by the female police chief. The reason is that she didn't want him talking. Years ago, when the female police chief was a rookie cop, that cop was a family friend. She went to him and told him what she had done by killing that girl. It was his idea to pin her as another victim of that serial killer and he put the stamp in her mouth to make it look like that and wrote it up as another kill of the serial killer. So, he covered up the murder by the lady rookie cop. Maybe he didn't deserve to die, but he should have come clean when questioned. He had to know the police chief was not going to go down for that murder and he knew the truth.