Underworld — Whitley Strieber

Flynn Carroll works for the most secret police unit on the planet, seeking the most brilliant and lethal criminals who have ever walked free: thieves and murderers from another world.

As part of a top secret CIA alien communications project, Flynn's unit has been tasked with tracking down rogue agents from the planet Aeon. While Aeon claims to be a free planet desiring open communications with humanity, Aeon criminals have committed a series of brutal and bizarre murders on Earth. Flynn has been forbidden to take lethal action against the alien murderers--but as the bodies begin to pile up, something must be done.

Flynn finds himself cut off from his team, struggling to unearth Aeon secrets while protecting Earthling civilians from the deadly creatures. But as Flynn gets closer to the truth, he finds himself facing not only some of the most dangerous and frightening criminals ever seen on Earth, but also questions about his own existence. In order to crack the case, Flynn must come to grips with the greatest mystery he has yet confronted: who--or what--is he?