My hunch is they've talked to enough people, friends, family,
co-workers, and others that they have eliminated accident or
intentionally disappearing.
I still think he was ambushed while out on the trail by an
unknown person committing illegal poaching of cows or game.
Sad to think about, but in these rugged parts, perps might
shoot someone rather than be caught and arrested. Especially
if perp already has a criminal record or is on probation or is a felon
carrying a gun.
Heck in my area, we've had several killings of witnesses prior
to testifying at trial. It's not that uncommon.