Lone Star Witness by Delores Fossen

Former military special ops officer, Slade McKenna, is on the most important mission of his life: to protect two very important targets in the rugged Texas Hill Country. His best friend, Marise Brennan, a veteran combat nurse who saved his life and his former commander, Colonel Vince Rosa, who taught Slade what it meant to be a hero. A ruthless killer is after them, trying to silence them for something the colonel witnessed—a murder with no body and no evidence. With Colonel Rosa and Marise dead, the killer could get away with the perfect crime.
As the attacks on Marise and the colonel escalate, Slade and Marise find themselves caught in a dangerous web. Along with the threats from the killer, the long-simmering attraction between Slade and Marise adds a personal dimension to the danger. Giving into that attraction could prove costly, jeopardizing not only the mission but also their lives. In a collision of love, duty, and honor, Slade and Marise face down the challenges of outsmarting a killer where their every move could determine the fate of all they hold dear.