I'm deep into my next WIP and I haven't figured out who the killer is yet. I'm on chapter 8 (I write in chapters) and am a consecutive writer meaning I start with chapter 1 and I'll end it at chapter 16 (I have the feeling this will be a 16-chapter book). I killed off somebody in chapter 2 (she was annoying anyway) but I'm not quite sure whodunnit.
I also killed somebody in chapter 6. It was too good an opportunity -- a writer's conference, an empty ballroom, so many murder weapons at hand -- I just had to do somebody in. I think the same person did both murders, but they may be unrelated.
I usually figure this out by chapter 12 or so. Then I go back and layer in the clues and establish the motivation. In one of my books (Candy, Corpses & Classified Ads, out in download now and in print in August), I didn't figure out who did it until chapter 14 -- man, I was starting to panic! The same is true for Autographs, Abductions, and A-List Authors (releasing next week): I was buzzing along killing people and all of a sudden ... WHOOPS. Who did it?
Now with my 3rd person books, I usually know from the get-go who did it, why, when, how, where ... but these first person books are almost like I'm channeling the story and I'm learning it as we go along, the same as the main character.
Hmm. There might be a plot there ....