Boy on a Tricycle

Chief constable of Oak Hill, California, Arnie Crockett, returns with his family from a vacation in nearby Yosemite only to discover that his close friend, Tom Whiting, the sheriff of Tulare County, has been murdered in his home. It turns out that this killing is only one of the three that appear to be linked.
Arnie and Tulare detectives fan out to hunt down the killer, with Arnie's only real clue being a small, old, B&W snapshot of a boy, about four, sitting happily on his tricycle being left inexplicably at one of the crime scenes. His search leads him all through Central California, from the little airport in Porterville to a clandestine pot farm in the Sequoias, to a flower shop in Tulare, and inevitably, to an unlikely killer.