As
@jepop says, the PCA, which was on January 5th.
However, I was looking through a very early timeline post, and LE mentioned that the weapon was probably some kind of knife, as early as the Nov 15 2022 press conf:
***UPDATED NOV 16, NEW INFO IN RED*** VICTIMS: EC = Ethan Chapin [IG / FB], a 20-year-old from Conway, Washington [1]; freshman, member of Sigma Chi fraternity majoring in recreation, sport and tourism management in the College of Education, Health and Human Sciences. [7]; school indicates...
www.websleuths.com
And I don't have a link for this, but: at some point about 4 months ago maybe, I read all the discussion threads from this case (for the first time).
People were speculating early on that a sheath must have been left behind, I think because at some relatively early point LE also mentioned the type of knife ("Ka-Bar")--the idea being, how do they know what type of knife if they don't have the weapon? They know because they found a sheath for that type of knife.
Hope this all make sense.