Last to Die (Rizzoli & Isles #10) by Tess Gerritsen

For the second time in his short life, Teddy Clock has survived a massacre. Two years ago, he barely escaped when his entire family was slaughtered. Now, at fourteen, in a hideous echo of the past, Teddy is the lone survivor of his foster family’s mass murder.

Orphaned once more, the traumatized teenager has nowhere to turn—until the Boston PD puts detective Jane Rizzoli on the case. Determined to protect this young man, Jane discovers that what seemed like a coincidence is instead just one horrifying part of a relentless killer’s merciless mission.