‘The Watcher in the Wall’ by Owen Laukkanen

His latest novel, The Watcher in the Wall, follows Kirk Stevens and Carla Windermere of the crime task force as they investigate a case that first seems to be a simple teen suicide.

But the detectives uncover an online club of unhappy teenagers vowing suicide, led by an anonymous presence.

Soon more victims fall – as they will continue to do unless the psychopath leader can be caught.