Completely Wrong
I looked through some of the other topics, but I don't know if anyone mentioned the fact that at the beginning of the movie, she was completely wrong.
First of all, she said that the killings were the work of a serial killer which, at that point, was completely untrue. It was his first murder and would remain the first for the next 2 years until he killed the Wendy girl. Even he said he didn't know why he did it, he just wanted to, which indicates that it was a spur of the moment decision. Which is exactly what the other cop was saying in the first few scenes before Eliza Dushku's character dismissed him.
Also, she made a big deal about the initials and the town all starting with the same letters which had nothing to do with the murders until he wanted to try and prove her right afterwards to help her get some credibility.
My point in all this is that she was a bad detective. We don't ever really get any scenes of her being a good detective really yet we're supposed to imagine that she is. Not one scene in the film does she do any detecting and she only stumbles upon the killer accidentally when she's at his house and sees the cat. If he had just hidden the cat in the closet or intentionally tried to kill her, she would've never found out anything.
This was a really bad movie. The twist was contrived and I don't understand why these movies always have to make it about someone closer to the protagonist. Sometimes there are just random murderers that you don't necessarily have to know. Also, the acting was atrocious, especially Eliza Dushku's. She stumbles through the entire movie, almost shoots a priest, breaks a poor innocent nurse's arm and then accidentally stumbles upon a killer.
Terrible.