For those who don’t care, it turns out that Carl didn’t kill Rachel or some of the other girls, but likely did kill the young mother he was charged with but not convicted of. It transpires that, because Carl is a very good photographer and observer of human nature and body language, he sees something in a photo of Rachel that no one else has. It’s this photo that leads Grace and Carl straight to the place where Rachel is buried, and they eventually discover what happened to her. The men, there are two of them, who were trying to kill Rachel, were both involved in the drowning of another young woman years previously. She was raped first. They feared Grace was getting too close to the truth during her investigations into Carl. Grace had been persistently blaming Carl for the murder.