If the story is correct, it appears that they have not eliminated the fact that he is a putz. Did he try to call back? Was he at all concerned that her call had ended as it did? If so, then why did it fall on a roommate to call the cops? This guy will (or should) be hating himself for the rest of his life.
My take is that this girl -- cute, blonde, and probably bubbly -- was friendly to a whackjob, who subsequently decided that they were an item. When she told him to go away, he did the typical whackjob thing, and grabbed her.
I certainly hope I'm wrong, and that it's another Elizabeth Smart type situation, but I don't think she's alive. However, given that it's North Dakota in winter, she's probably not buried, or if so, not too deep.