Knock knock

"The #1 international-bestselling thriller that tells the electrifying story of a police inspector and a former criminal informant in a race against time as they attempt to unravel past and present secrets. Seventeen years ago, Criminal Inspector Ewert Grens was called to the scene of a brutal crime. A family had been murdered, and the only survivor--and witness--was the five-year-old daughter. The girl was placed in the witness protection program, and the case went cold, but years later, Grens is still haunted by the seemingly random slaying, and the little girl who was spared. So when he learns that the apartment where the crime occurred is now the scene of a mysterious break-in, Grens immediately fears that someone is intent on silencing the only witness." --