The biggest issue here is the failure to investigate the fourth boy, likely Tobin, and question him.
It is established, particularly as a function of BK's social life, that Tobin provided a primary venue for multiple group activities, particularly on July 1's entry, upon which they were consuming alcohol as a group of five boys, and potentially unnamed additional guests.
Literally the first thing that went through my head when she didn't know where the house was or the identity of the fourth boy was "if we can make a list of potentials for the "fourth boy", we can isolate potential crime scenes." While there is certainly no evidence of an attack from 30 years ago, this DOES allow corroboration of details, as it would establish that 4+ persons met in a house with a narrow stairway and consumed alcohol there. It would also allow the investigator to question Tobin as to whether additional female guests were ever brought into such a place. This information would corroborate Ford's testimony, particularly in light of BK's letter which indicates that he DID in fact interact with girls from Holten Arms. It would also mean that BK, Judge, and PJ all perjored themselves.
This was not even attempted, largely because it was not authorized. I'd bet my left nut that this went through more than just my head. This would take only a day or two to investigate and put a report together on. Any FBI investigation that does not actually investigate whether the claims are consistent with reality is no investigation at all. Any statement of fact which may be supported by historic record and first hand experience should be tested. But no such elements of testimony were investigated. They only questioned a couple people who had a vested interest in lying.
How can we accept this investigation when they didn't do what was literally the most obvious thing an honest investigation would do?