The Sixth Man by David Baldacci
After alleged serial killer Edgar Roy is apprehended and locked away in a psychiatric unit, private investigators Sean King and Michelle Maxwell are called in by Roy's lawyer-an old friend of Sean King- to look into the case. But their investigation is derailed before it begins: while en route to their first meeting with the lawyer, King and Maxwell discover his dead body.
A rash of terrifying events begins to unfold and it is up to King and Maxwell to uncover the truth: is Roy a killer or not? But the more they dig into his past, the more they are bombarded with obstacles, half-truths, and dead ends that make filtering the facts nearly impossible. As each new theory brings a new revelation, King and Maxwell will be pushed to the limit. Could this deadly case be the one that leaves the duo permanently parted?