"he somehow found out what was in the investigation files"
Good point. Also, I read on one of these threads that Karr had corresponded with Jonbenet's grandparents under the guise of writing an investigative piece on the murder.
I had not heard that little tidbit. Even if true, I wouldn't think that the grandparents would have given away any classified information regarding the murder of their granddaughter. However, some elderly folks can be victims too.
I heard it mentioned that this guy was writing a book. That's why I got to thinking that IF that is true, perhaps he managed to bribe someone in the police department, etc., OR perhaps gained the trust of, or became friends with someone who had access to classified information and got them to say more than they should have. Stranger things have happened.
The police don't tell the family everything. Stuff that is withheld would be withheld period. The Grandparent would not know anything.