The Ice Queen

Nele Neuhaus's The Ice Queen is a character- and plot-driven mystery about revenge, power, and long-forgotten and covered up secrets from a time in German history that still affects the present.
The body of 92-year-old Jossi Goldberg, Holocaust survivor and American citizen, is found shot to death execution-style in his house near Frankfurt. A five-digit number is scrawled in blood at the murder scene. The autopsy reveals an old and unsuccessfully covered tattoo on the corpse's arm—a blood type marker once used by Hitler's SS.
Pia Kirchhoff and Oliver Bodenstein are faced with a riddle. Was the old man not Jewish after all? Who was he, really? Two more, similar murders happen—one of a wheelchair-bound old lady in a nursing home, and one of a man with a cellar filled with Nazi paraphernalia—and slowly the connections between the victims becomes evident: All of them were lifelong friends with Vera von Kaltensee, baroness, well-respected philanthropist, and head of an old, rich family that she rules with an iron fist.
Pia and Oliver follow the trail, which leads them all the way back to the end of World War II and the area of Poland that then belonged to East Prussia. No one is who they claim to be, and things only begin to make sense when the two investigators realize what the bloody number stands for, and uncover an old diary and an eyewitness who is finally willing to come forward.