Unknown Threat by Lynn H. Blackburn

US Secret Service Special Agent Luke Powell is lucky to be alive. Three of his fellow agents have died in unusual circumstances in the past ten weeks. Luke is devastated by the loss of his friends and colleagues, and his inability to locate the killer feels like a personal failure. He’s an expert at shielding others, but now the protectors are in need of protection. FBI Special Agent Faith Malone is driven to succeed and confident in her ability to solve every case she’s assigned. She’s been put in charge of the investigation into the unprecedented attacks, and with Luke’s life in danger, the stakes have never been higher. But it’s hard to know how to fight back when you don’t know who the enemy is. As more agents are targeted, Luke and Faith will have to work together to bring a killer to justice and prevent any more names from joining their fallen brothers and sisters on the Secret Service Wall of Honor.