The Aosawa Murders by Riku Onda, Alison Watts (Translated by)

They are having a big family event:  Three generations are all having birthdays and it's a big party day.  When some children show up later to join in, they find almost everyone dead...

Bitter Lemon Press and Edelweiss allowed me to read this book for review (thank you).  It will be published February 18th.

There is one lone survivor.  She's blind and she didn't see what happened.

Someone delivered sake and pop, all of it laced with cyanide.  The cop who questioned her feels she must be responsible but someone had to help her.

They look for the delivery boy and finally find him but he's committed suicide.  The cop still thinks she was part of it but the upper management declares the dead young man as the culprit and closes the case.

The girl next door interviews all she can and writes a book about it. It's like the crime that would never end.  As time goes on, more clues surface but the answers aren't clear.

This is a long book that moves slowly.  You learn about the characters feelings and their thoughts on the murders.  I didn't think we got a cut and dried solution but it will make you think.