Blood Trail — Kristine Kathryn Rusch

Detective Zack Wheldon solves cases. The tough cases no one else figures out. So, when the FBI comes seeking his help, he must decide which to choose: the serial killer he desperately wants to catch or the as-yet-unknown cases that will grow cold in his absence. The FBI promises big things, more closed cases, more criminals behind bars. If he can trust the feds. If he believes in their methods. If he really wants to test his own resolve.
“Kristine Kathryn Rusch's crime stories are exceptional -- both in plot and in style.”
-- Mystery Scene Magazine