The Hypnotist by Lars Kepler

In a small Swedish town a family (a mother, a father, and their daughter) was murdered in a particularly sadistic way. There is one survivor: the son and brother of the slain persons. He was also terribly injured, he was cut several times with a knife. Joona Linna, a Detective Inspector, insists a dubious alternative: she talks Dr. Erik Maria Bark into hypnotise the boy to make him regain his memories about the murderer. However, as he does so indeed, something unthinkable occurs…