The Absent One by Jussi Adler-Olsen

Detective Carl Morck and his assistant, Assad, decide to explore a case that's over twenty years old. Teenage siblings Lisbet and Soren were beaten to death in the family's summer cottage, but it wasn't until nine years after their murders that Bjarne Thogersen confessed and went to prison.  Carl becomes increasingly skeptical about Bjarne's guilt when he finds that Bjarne has become very wealthy while in prison, despite coming from a modest home.  At boarding school, Bjarne ran with a wild crowd that were suspected at the time of being involved in the murders, but nothing was ever proven until Bjarne's admission. Since then, Bjarne's classmates Ditlev Pram, Torsten Florin, and Ulrik Dybbol, have become famous and wealthy themselves, but an evil streak runs through all three.  The reader knows they were probably involved in the murders, as was Kirsten-Marie Lassen, another group member (and the only female)--now known as Kimmie and living on the streets.  Although Kimmie was in the circle, she wants to exact revenge on the men for their past misdeeds towards her.  The question remains: will Carl and Assad be able to find proof that the friends are guilty?  The Absent One is the second book in the Department Q series and is not as strong as the first.  Because the reader knows the culprit early on, there is not enough story to sustain a four hundred page book.  I will, however, seek out the next book in the series because I like the character of Carl Morck.