Icebergs

A great mystery is like an iceberg. We find most of the truth below the surface.

The trick of writing a great mystery/suspense is to tell the reader everything they need to know without them realizing you told them everything they need to know.
Provide clues that double as representing something important to the overall plot.

For example, in my current novel, "Mr. REE, Detective," a teenage girl finds a barrette in the elevator, which makes her feel pretty and stops the annoying comments by her dad's girlfriend to brush her hair. It is however a major clue to the identity of the killer and isn't even a barrette at all!