Fuurinkazan: Shadow and Lightning's Conclusion

Kansuke Yamato reveals that Shigetsugu hooked his fishing rod onto the railroads power lines resulting in his death. He shows them Shigetsugu's diary which explains that the four victims were the cause of Kuroto Kai's death and deduces that Shigetsugu was the culprit who murdered the three victims before committing suicide.

Synopsis excerpt via Kitsu.