Kiseijuu Reversi

Three dismembered bodies were found in an abandoned basement parking garage, sliced limb from limb while they were still alive...and if the boy who discovered them is to be believed, the murders occurred in an impossibly short window of time. Nothing is adding up for veteran detective Fukami of the East Fukuyama police department, and his gut is telling him the boy is hiding something. In fact, he's quite certain that Tatsuki Hirokawa saw exactly what happened, and who did it. But what the boy knows will go far beyond even his wildest nightmares...

(Source: Kodansha USA)