Knights of the Blood Series by Katherine Kurtz, Scott MacMillan

Captain John Drummond of the LAPD is investigating an unsolved murder from more than thirty years ago-six unidentified men, bearing Nazi SS tattoos, drained of blood-that echoes a series of unexplained deaths around the world. But these murders go back further than he expected-to World War II, to the time of the Crusades, and to a group of Knights Templar who have lived for centuries...

A group of Knights Templar and a cadre of Nazis have been affected with the same affliction-vampirism-and have fought one another since the closing days of World War II. Now, LAPD detective John Drummond knows about their secret war and will stop at nothing to end it...