The Triggerman's Dance by T. Jefferson Parker

When a woman is mistakenly assassinated by a sniper from a white supremacist militia, her death brings together her FBI agent fiancee and her journalist lover, John Menden. The two team up to exact revenge by destroying the militia group, known as the Freedom Ring, with Menden infiltrating the group's Liberty Ridge compound. When he falls in love with the commander's daughter the plot becomes tautly complicated. In a scenario grounded in events on the right-wing fringes of U.S. society, Parker engineers a grandly violent climax to assuage all Menden's macho hatred.