Parting Shot (Promise Falls Trilogy 04) by Linwood Barclay

After a tragedy rocks the community of Promise Falls, Cal Weaver is asked to investigate the threats being made to the accused's family. He's heard all about Jeremy on the news: the young man who drank too much, stole a Porsche and killed a girl, and who claimed afterwards not to remember a single thing. It seems an open and shut case, but for reasons he can't explain he accepts the job. Soon Cal finds himself caught up in a vicious revenge plot, and it's only a matter of time before threats turn into action . . .
A gripping thriller packed with scandal in a small town, from the master of the twist you never saw coming.