Watcher in the Woods by Kelley Armstrong

The secret town of Rockton has seen some rocky times lately; understandable considering its mix of criminals and victims fleeing society for refuge within its Yukon borders. Casey Duncan, the town’s only detective on a police force of three, has already faced murder, arson, and falling in love in the several months that she’s lived there. Yet even she didn’t think it would be possible for an outsider to locate the town and cause trouble in the place she’s come to call home.
When a US Marshal shows up demanding the release of one of the residents, but won’t say who, Casey and her boyfriend, Sheriff Eric Dalton, are skeptical. And yet only hours later, the marshal is shot dead and the only possible suspects are the townspeople and Casey’s estranged sister, smuggled into town to help with a medical emergency. It’s up to Casey to figure out who murdered the marshal, and why someone would kill to keep him quiet—before the killer strikes again.