The Silence of the Dead - The Charlie-316 Series, #6

Every city has secrets. Some are evil. A killer stalks Spokane, Washington, with his history of victims stretching back fifty years. It seems an unlikely truth, but Major Crimes Detective Wardell Clint is driven to prove it. After discovering tenuous links between many unsolved murders, Clint develops a theory which forces him deeper into the past. He must wade through generations of cops, witnesses, and survivors to discover a truth hidden behind privilege and cover-ups.
His improbable guide on this journey-retired Police Chief Robert Baumgartner-may be his greatest ally or most dangerous opponent. No matter how long it takes, Clint won't give up. His job-his duty-is to speak for the dead. And they've been silent for too long.