Lieutenant Fox (spoilers)
So, Jenny runs into Lt. Fox, who tells her that he's checking up on her because her parents asked him to.
Later, Jenny calls her parents to get his phone number and finds out they didn't ask him to check up on her at all. So Jenny believes Fox is the person who is running around killing everybody.
Later, when she's in the warehouse, she can hear Fox calling out to her to see if she's okay. When it's revealed Gavin was the killer, Fox comes in and shoots Gavin.
That's all well and good except, none of this explains why Lt. Fox really was following Jenny in the first place or how he found her in the warehouse at the end of the movie (she never got his phone number). So he wasn't the killer, but that doesn't mean he's not a bit of a creep. And the movie never bothers to explain it.