The Last Widow (The Will Trent Series, Book 9) by Karin Slaughter
It begins with an abduction. The routine of a family shopping trip is shattered when Michelle Spivey is snatched as she leaves the mall with her young daughter. The police search for her, her partner pleads for her release, but in the end...they find nothing. It's as if she disappeared into thin air.
A month later, on a sleepy Sunday afternoon, medical examiner Sara Linton is at lunch with her boyfriend Will Trent, an agent with the Georgia Bureau of Investigation. But the serenity of the summer's day is broken by the wail of sirens.
Sara and Will are trained to help in an emergency. Their jobs - their vocations - mean that they run towards a crisis, not away from it. But on this one terrible day that instinct betrays them both. Within hours the situation has spiralled out of control; Sara is taken prisoner; Will is forced undercover. And the fallout will lead them into the Appalachian mountains, to the terrible truth about what really happened to Michelle, and to a remote compound where a radical group has murder in mind...