Bad intentions - Karin Fossum

Early one September three friends spend the weekend at a remote cabin by Dead Water Lake. With only a pale moon to light their way, they row across the water in the middle of the night. But only two of them return, and they make a pact not to call for help until the following morning.

Inspector Sejer leads the investigation when the body is discovered. He is troubled by the apparent suicide and has an overwhelming sense that the surviving pair has something to hide. Weeks pass without further clues, and then in a nearby lake the body of a teenage boy floats to the surface.