The Case of Disappearing Detective Boys

It appears that following the murder of Komiyama, three people stepped forward as the murderer. However, which one of the three is the actual murderer? Katsumata Kengo, Niikura Yumiko, and Hoshino Hariyuki all had a motive, and when questioned by the police, all three admitted to the murder. Katsumata Kengo struck him over the head with a vase after being angered by the man and moved his body into the utility closet. However, Hoshino Haruyuki likewise admitted to murdering the man after being told that Yumiko would never be his by Komiyama and then hit him over the head with... a vase. While at school, Niikura Yumiko admits that Komiyama made a pass at her and came after her at the school and getting frightened, struck him over the head... with a vase. So who is the actual killer?!? It's up to Conan to help Mouri Kogorou and the police solve the case of the four times killed man.