The Watcher in the Wall — Owen Laukkanen

Kirk Stevens and Carla Windermere of the joint BCA-FBI violent crime task force have handled shocking cases before, but this one is different. Stevens's daughter, Andrea, is distraught over a classmate's suicide, but what the two investigators find is even more disturbing -- an online suicide club of unhappy teenagers, presided over by an anonymous presence who seems to be spurring them on. Soon, it becomes apparent that the classmate wasn't the first victim -- and won't be the last, either, unless they can hunt down this psychopath once and for all.