Shinsuke Akiyama is the culprit. He faked his own death using Takeda's lifeless body (whom he first strangled, then he decapitated) as a decoy, and was aiming to kill Saegusa and Yamato too. His motive is that years ago, Yamato and his companions's mistakes led to a serial killer terrorising Nagano before being shot down, one of his victims being Akiyama's beloved younger sister.
