Carolina Moon by Nora Roberts. One of her better standalone romantic thrillers, IMO.
I found the secondary couple more interesting than the primary couple in this case, and I don't know if the hint was too obvious or if I simply had one of my hunches, but I had the killer pegged from his first appearance.
I have come to the conclusion that I like Nora's supernatural pure romances better than her non-supernatural pure romances, and her non-supernatural romantic thrillers better than her supernatural pure romances. The fantasy romances, thrillers or not, I can take or leave.
Carolina Moon is one of her supernatural romantic thrillers, but one without magic or fantasy elements, for which I am glad , because they would have ruined the story.