This word is acceptable for play in the UK dictionary that is being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

n. A type of detective story in which the focus is not on who committed the crime, but what were their motives for committing it.

"From Hell, a book designed to not be a "whodunit," but to be a "whydunit" and explore the London around the Jack the Ripper crimes, the mentality of the killer, the reactions, an exploration of violence, and of many other things."

whodunit    howdunit