Write to find out.
I myself, as I’m writing, don’t know who did it. The readers and I are on the same ground. When I start to write a story, I don’t know the conclusion at all and I don’t know what’s going to happen next. If there is a murder case as the first thing, I don’t know who the killer is. I write the book because I would like to find out. If I know who the killer is, there’s no purpose to writing the story.