Last to Die by Tess Gerritsen

Detective Jane Rizzoli and Forensic Pathologist Maura Isles find themselves locked inside a high-security boarding school along with the students. They are told it's for their own safety.

The school has been set up to provide a place of sanctuary for chidren who have lost their families in violent circumstances. But now there has been a murder within the grounds.

Does the threat come from outside...or from within?