Numbers Game - J. P. Jordan - Bog

At a fundraiser in his honor, Senator Anthony Hecht is invited to a private room by a young, auburn-haired beauty. Minutes later, the politician dies of an apparent heart attack, but when police arrive, they discover the man's forehead has been marked with the number seven. And, his female friend has disappeared.
Detectives Chuck Nowitzke and Anissa Taylor---first introduced in the popular novel, All In---are assigned to investigate the high-profile death. Their inquiry leads to several obscure cases from years prior involving men who died in similar manners. Each with a number on their body, but with no official determination that any of them were murdered.
Puzzled by the circumstances and driven to find a link between the deaths, the detectives must also solve a decades-old missing persons case in what appears to be a growing cover-up by a possible serial killer.