Graham, Heather. Blackbird trilogy #1

"When darkness falls, there's nowhere to hide. Four bodies have been discovered along Europe's riverbanks, placed with care-and completely drained of blood. Pinpricks on their throats indicate a slender murder weapon, but DNA found in the wounds suggests something far more sinister. Tasked with investigating, the FBI recruits Agents Della Hamilton and Mason Carter to Blackbird, an international offshoot of the Krewe of Hunters. If you want to catch a vampire killer, you need agents who can speak with the dead. The pair travel to Norway, where the shadowy forests of Lillehammer reveal a gruesome scene. The killer is thirsty for more victims, and the bloodless trail soon leads Della and Mason to a group that believes drinking blood is the key to immortality. To catch the culprit of such an intimate crime, the agents will have to get close. Mason's already lost one partner; he's not ready to risk Della as bait. But sometimes justice requires a sacrifice The Blackbird Trilogy Book 1: Whispers at Dusk Book 2: Secrets in the Dark Book 3: Cursed at Dawn."