If they tracked him closely he must have discarded the murder weapon immediately after the carnage. I would check local dumpsters and POSSIBLY where those dumpsters were emptied, along with local brush/bush locations. Monumental task and they’ll probably never find the knife.
There are a lot of bridges between Idaho and Eastern PA. That knife is at the bottom of some Midwestern stream.
There are still a lot of unknowns out there, but there seem to be some pretty valid news items regarding his near obsessive vegetarianism. Two of the victims worked at one of the best known vegan restaurants in the area.
Assuming this is how the victims came to be known to him, this was likely a fixation/obsession killing, and if that's accurate, then the weapon was more than that...it was his means of taking control and possession of his target(s)...and if that's the case it was a substitute for his, um, manhood.
Purely speculation on my part, but if correct, he may have been a lot less prone to dispose of the knife than somebody who stabbed somebody in a mugging or typical street assault.
If they tracked him closely he must have discarded the murder weapon immediately after the carnage.
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He went back to class till the semester ended. Lots of time and places to dump the knife. When did they start tracking? If immediately, they had him on the radar before he left. The didn’t search his room and take evidence till after the arrest. Did they search it secretly before that? A public search would have alerted the whole community.
I am going to say they didn’t track him real time. Maybe went back to videos along his route to piece it together after they got any DNA, then vehicle registration....