I am writing a novel and it goes something like this: In the first half of the novel, the protagonist knows who is doing the killing, but they are trying to make sense of it as in "how are they doing it?" and "why are they doing it?" In the second half of the novel, the protagonist finds out there's an object that can stop the killing altogether, so the main question is "where is the object?"
Would this story classify as a mystery?
Can you do it? Sure. As opposed to what? Do you think the Mystery Novel Police will arrest you for breaking the rules of the genre?
The question is not, "Am I allowed to do this?", but "Does it make a good story?" If the answer to the second question is "yes", then do it. If as you write this story you discover that it's not working, that the story is boring or confusing or whatever, then drop it and try something else.
Have you ever seen the old TV series "Columbo"? Each episode was a murder mystery, but they were not really "who done it"s. Each episode began showing the audience the murder happening, so the audience knew from the start who the killer was. And we were usually given to understand that the detective quickly figured out who the killer was. The mystery was always, "how can he prove it?"
Most mysteries don't exist in isolation, but rather they are manifold, take one of my favourites, Roanoke, there are several mysteries within the mystery of the Roanoke disaster:
where did the colonists try to go?
where did they actually end up?
when did they leave?
and most "interestingly" what on Earth does "CROATOAN" mean?
So multiple related mysteries certainly have a place in a single piece.
As long as both mysteries are resolved, I don't see a problem with it.
+1 Ash for Columbo. Also, there was recently some miniseries on TV about a woman, a young mother. In the opening, she inexplicably attacks a man and murders him in front of many people. The mystery is not who did it, or how to prove it, but why: Even she does not know why she did it, and has no motive.
I did not watch it (short on time for other professional reasons), but it is an interesting premise.
So yes, investigating "how and why" can be interesting, and once "how and why" is figured out, investigating how to stop it can be the rest of the story.
Yes, that can still be called a mystery. Much of the motive in investigating any murder is punishment, but a healthy part is also to get a danger to society in jail, in order to prevent future murders. Presumably the first murder is the one they hesitated on the most. This is just a logical extension of that same motive for the investigator.
