The Absent One, by Jussi Adler-Olsen

Carl  Mørck, head of Copenhagen's cold case Department Q, now has two very capable, very odd people working for him, Assad, a civilian police employee who doesn't seem to get the difference between being a police officer and a civilian aide, and Rose, who is bossy, moody, and snippy, but wonderfully resourceful and capable.  They start out investigating the 20-year old murder of a brother and sister and find it linked to a long list of brutal attacks.  Powerful forces are out to stop them, by any means necessary.  The good guys are a hoot, the bad guys a little over the top. The Danish title was Fasandræberne, meaning Pheasant Killers; the UK title was Disgrace; and the US title The Absent One.  There's something a little odd about that. I want to read his last two:  Message in a Bottle and Journal 64.