The explanation of how the killer used a string-like object from a cassette tape to place the key into the victim's hand is a classic Conan locked-room puzzle solution.
The investigation quickly shifts focus to three primary suspects, each with a plausible motive tied to the author's wealth, literary legacy, or personal grievances. As Sato and Takagi interview the suspects, the underlying tension regarding Sato's ring continues to simmer, threatening to distract Takagi from critical clues. Anatomy of the Mystery: Conan’s Brilliant Deduction detective conan episode 487