Mouryou no Hako
I'd just like to mention some of the obvious call outs this series has to the mystery genre. Besides Sekiguchi openly mentioning discuss tropes of the mystery genre there is also the presence of numerous 'investigators' in the series.
We have Detective Kiba, who is your traditional police detective who investigates people, interviews important characters and looks for inconsistencies.
We have the journalists type investigators in the form of Atsuko and Toradochi.
We have Enokizu Reijirou as the slightly rude, slightly full of himself private detective that appears to be able to intuit things very easily.
Finally we have Kyougokudou as the all knowing character who as already got everything figured out. He possesses a worrying amount of insight into the case.
I'm not quite sure where Sekugichi fits into this team of investigators, or why the series has quite so many characters investigating this mystery.