I can't not remember the name of the investigator you are speaking of but I do remember this.
He did indeed quit and persued the case on his own. He also described how members of the Boulder Police and prosecutors office told the investigators not to look into other evidence since the "family did it and prove it".
It's not ridiculous or unusual in cases like this to look to the family or a family member as the guilty party. The risk in this thinking is to overlook evidence that points to another person where the actual killer gets to walk away.
I remember when the Jessica Lunsford cases first broke. Speculation was that the grandfather committed the crime. If the police had kept on that line of thinking, had not investigated other evidence found at the scene and completely persued the Grandfather, Couey wouldn't have been identified, arrested and currently awaiting trial for her brutal torture, rape and death.
Lou Smit