Redemption by David Baldacci

Decker is visiting his hometown of Burlington, Ohio, when he's approached by a man named Meryl Hawkins. Hawkins is a convicted murderer. In fact, he's the very first killer Decker ever put behind bars. But he's innocent, he claims. Now suffering from terminal cancer, it's his dying wish that Decker clear his name.
It's unthinkable. The case was open and shut, with rock solid forensic evidence. But when Hawkins turns up dead with a bullet in his head, even Decker begins to have doubts. Is it possible that he really did get it wrong, all those years ago?