Rat catcher

"According to underworld legend, the Rat Catcher is a peerless assassin who specializes in silencing mob snitches. There's just one problem-- he doesn't exist. At least, not according to the U.S. Marshals who oversee the Witness Protection Program. Now there's a pile of dead bodies in a burning safe house outside El Paso. The Rat Catcher has finally slipped up, and a washed-up FBI agent has one last chance to hunt him down before he disappears forever. But as the two men spiral in towards each otehr in a deadly game of cat-and-mouse, each of them hides a deadly secret from the other-- a secret that could destroy them both"--P. of cover.