Young Shunen is the killer... as well as the younger brother of the deceased monk Chunen. Some time ago Chunen was in love with Tenei's granddaughter Kikuno, who not only was the heiress to the temple but had just accepted to marry another Buddhist monk from a very important monastery. Tenei was so upset at such an affair, that when he learned that Chunen and Kikuno were about to elope, he killed Chunen using exactly the same trick that Shunen later used on him (which is described below), managing to strangle the guy and then set the scene in such a way that it looked like a perfect killing by the Mist Goblin.
Shunen was always suspicious of Tenei and the other monks, so he hid his identity and entered the monastery to gain their trust and learn what had happened to his brother. He spent six months quietly snooping around, managing to more or less figure out how the murder was commited, but not the identity of the killer.
Shunen confronted Tenei right after the Mouris and Conan had gone to bed, having noticed how he had openly spoken about the Chunen incident when prior to that no one had ever menttioned it; under the effects of alcohol, Tenei told Shunen everything. The young novice begged his superior to turn himself in, but Tenei refused and said he'd rather let everyone think the Mist Goblin took Chunen's life, since such a "mystery" had let him raise the fame of the monstery. Shunen then lost it and used his newfound knowledge to kill Tenei in the same way Chunen died: he grabbed a rope, wrapped it around Tenei's neck, and pulled it as if he was carrying a heavy bag on his back (the "bag being, of course, Tenei).
After recovering his senses and seeing that he had murdered Tenei, Shunnen decided to use the same tricks Tenei used on Chunen to throw suspicions off him: placing the body in a lifeboat, using the nearby cascade water to slowly fill up the room, hanging Tenei's body from the ceiling, and then breaking the wooden wall.
