Inspector Francis X. Flynn was introduced in the second Fletch novel by Gregory Mcdonald and he got a book of his own the next year. He's a terrific character, a laconic, sarcastic weirdo with a musical family, who treats the theft of his son's violin as every bit as important as the passenger plane that explodes almost atop his house as it's lifting off from Boston's airport.
With a federal judge on board along with several other notable celebrities, there could be any number of motives for the mass murder, including a local cult that preaches killing as the only way to curb the planet's population, but the real question for Inspector Flynn is who is this detective, and why does the captain assign him to work with the feds on the case when the Boston police department does not actually recognize the rank of inspector?
This is just a hugely fun book. I love Flynn's relationship with his suffering sergeant. At one point, Flynn sends his teenage sons undercover to do a little work for him, and sends the sergeant to chase them around to make it look good. It goes exactly as Flynn planned it, but not at all like how his sergeant did. His recounting of what happened is just about the funniest thing I've read in months. Highly recommended.