Violets are Blue by James Patterson (2001) little Brown & Company

Alex Cross's partner is found brutally murdered in her own bed.  But he responds to his friend and handler at the FBI, Kyle Craig, telling him about a violent murder of two military personnel in San Francisco while they were out for a run.  They appear to have been attacked by a large animal and then hung, naked, by their heels.  Their blood has been drained, but too little of it is on the ground.

Alex meets a detective, Jamila Hughes, who is sure this is not the first such murder.  The coroner has called in an animal bite specialist who identifies the bites as being from a tiger.  Only tigers devour their prey, they don’t back off after a few bites.

Further investigation in San Luis Obispo, Los Angeles and San Diego leave the detectives with the impression that there are people living lives as though they were vampires – even to the extent of having their incisors lengthened.\

A boy, who has killed his parents and is hiding on a ranch with other kids, bites Alex when he is captured.  The bite makes Alex very sick and while confined to home, his finds a pattern on the east coast and possible suspects.  A pair of magicians.

But as they close-in on the pair of killers, they are killed themselves.  The act takes them back to the west coast and they finally track-down the new killers.  This turns out to have been a war to see who would be known as the ‘Sire’ of the vampires.

When Alex gets home, he is attacked by his nemesis, the Mastermind.  But Alex manages to get the upper hand and discovers it is his friend and handler, Kyle Craig.