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.
The culprit is Saya Kitami, and she is actually 29 years old.
Afterwards, Hyoue Kuroda arrives on the scene, announces that Kiyonaga Matsumoto has been promoted, and that he will be taking his place. Conan looks on in shock, and Ai grips his shoulder in fear, as Kuroda asks for Conan's assistance in his new position.
