I'm a librarian who loves anime, manga, and reading a wide variety of genres.
I think this is where I stopped. So far we have two bodies, one crime, and a building about to be moved over a few feet while its inhabitants are still inside ("Trust us, you won't feel a thing, it'll be so smooth that you could have a glass of water in your room and not a drop would spill" haha, no, I'd totally go somewhere else while this was happening).
I'm a little confused about the timeline, but it looks like a woman in the building decided to take part in a kidnapping with her male partner. Because only women are allowed in the building, the guy pretended to be her female cousin. They kidnapped a half-American (or British?), half-Japanese boy and asked for ransom money, but something went wrong and the boy ended up dead. They buried his body in the building's basement and sealed everything up with cement. That was some time ago. At some later point (I think prior to the building move?), the male partner was run over and killed while dressed as his partner's female cousin, leading to much confusion on the part of the police.