After midnight, two FBI agents slipped into the backyard of a trim white bungalow on the outskirts of St. Joseph, Mo., tried the locked doors of the darkened house, inspected'the yard until their flashlights' beams came to a cluster of wilting yellow chrysanthemums by the back porch. Shoveling the flowers aside, the agents started digging. As the sun came up, they stopped and waited until a workman, Claude James, came along the street. They gave him a job: digging for the body of six-year-old Bobby Greenlease, murdered by kidnapers who had planted the chrysanthemums over his grave.
It took a...