Why are mysteries so popular? We read them, watch them on television, and follow them on the news. We want to know what, how, and why. We want to solve the mystery and we want the bad guys to be punished (most of the time, at least).
When writing a mystery there are some crucial elements to keep in mind. The reader has to care about the characters and about their goals and motivations. The reader has to be drawn into the story and the tension has to escalate. The story has to answer their questions--but not always in the way the reader expected.
I've read and watched a lot of mysteries and I've always wondered how these "brilliant" detectives make their incredible intellectual leaps. I'd see the red handkerchief on the floor, pick it up, and toss it in the laundry; it wouldn't automatically tell me that the cousin killed the butler with a bread knife in the bathroom.
The first mystery I wrote (as yet unpublished) had a pair of bungling detectives who solved the crime only by accident. It actually won NYC RWA's "Love and Laughter" contest a few years ago. Maybe I'll iron it out and publish it. We'll see.
In the meantime, the mystery I'm working on now has a serial killer on the loose. At this point--not quite halfway through the book--the police are starting to investigate what looked like "accidental" deaths. The reader probably has a good idea of who the killer is.
My hero and heroine? Well, they don't have a clue.... Yet.