What was that cops problem (other than being poorly written character)?

I know some cops can be incompetent and make up their minds about a case too soon, but why was he so convinced that Sara killed her friends? They'd gone into a totally unexplored cave system, something that's very dangerous. It's not hard to believe they came to a bad end. Sara is pretty beaten up and looks genuinely traumitized.

He acts like he's very suspicious, maybe even convinced she did it, though. He even forces her to come along with the little search party when they go to the cave.

It's never explained why. Maybe I missed something, but there's not a single bit of evidence or anything to cause him to think that, other than some blood on her clothes, but that doesn't mean much. It could have gotten there when she tried helping an injured friend.