800 dude wrote:
Two reasons, both above.
It's super weird/unusual that her boyfriend just shows up without her and utterly refuses to say a word about what happened to her. Doesn't matter that he probably killed her because there's zero evidence, and he can't be compelled to talk. Most people in his situation would talk to the police and eventually get caught in a lie, plus they'd give enough info to at least narrow down the place/time of death, which makes it a lot easier to focus investigative resources.
Also she's young and blonde, which automatically makes her disappearance national news.
So the perfect murder is to drive someone to Wyoming, kill her, dispose of the weapon and drive home? He's in some deep sh*t.