This Fallen Prey by Kelley Armstrong

When Casey Duncan first arrived at Rockton, the off-the-grid, isolated community built as a haven for people running from their pasts, she had no idea what to expect. There are no cell phones, no internet, no mail, and no way of getting in or out without the town council’s approval. She certainly didn’t expect to become the town homicide detective. But, the very last thing she expected was for the council to drop a dangerous criminal into their midst without a plan to keep him imprisoned. And she never thought that she’d have to be responsible for him.

The longer Oliver Brady stays in town, the more people seem to die around him. When evidence begins piling up that someone inside Rockton is working as his accomplice, Casey races to figure out who exactly Brady is and what crimes he’s truly responsible for committing.