The Absent One, written by Jussi Adler-Olsen, is the second book in the department Q series. Carl Morck is a police detective who has been relegated to a dead file department, reviewing old unsolved cases.
In this book, a group of school classmates have a history of physically attacking strangers, beating them senseless, and in some instances, killing them. Flash forward and the core of the gang are wealthy, influential and just as craven. When carl starts to make headway on an old double murder, they use their influence to try to detail him.
Well written, entertaining with intriguing characters.