Location of the knife.
LE has said they do not have the knife. We know that his apartment in Pullman was searched and his parents house in PA.
All speculation, moo:
I think he either immediately disposed of the knife, tossed it out the car window, into a body or water, random dumpster or trash can, whatever, on his way home to his apartment after the murders.
Or, more likely, imo, he kept it and hid it somewhere he believes is clever. I'm reminded of the
Mel Ignatow case. Ignatow had kept some of his victim's jewelry (that was quite valuable) and there were pictures taken when the crime was occurring. The undeveloped film was also there. It wasn't found until after Ignatow was acquitted, when his former house had some work done. Taped up into duct work that had been covered with carpet.
I think BK may have documented the crime or the aftermath. Maybe a cheap digital camera, or even a burner phone with a camera. I think he may have kept souvenirs that he's stashed, expecting/hoping someone finds them five, 10, 20 years from now. He possibly has some HVAC knowledge and think of all the buildings he may have been in since the murders. Could be anywhere. Taped under a urinal at a rest stop on I-80 for instance. Unlike Ignatow, I think BK would want them found at some point, though.
I think he's going to play games with this whole case. I was really surprised that he waived extradition. I think his comment, if accurate, "Was anyone else arrested?" and telling his extradition attorney that he looked forward to being exonerated is all a big mind game to him. I think he sees this as his greatest research project, and will come up with what he believes is a brilliant defense. Will he say his DNA was there because he was having some sort of relationship with one of the victims? A secret tryst?
I hope I'm wrong, and he just pleads to spare himself the death penalty, so the families and the community don't have to go through a trial. I think BK will do everything possible, though, to make this a spectacle for his own entertainment.
all MOO