Blindsighted by Karin Slaughter

Police chief Jeffrey Tolliver - Sara's ex-husband - is in charge of the investigation, and when a second victim is found, crucified, only a few days later, he has to face the fact that Sibyl's murder wasn't a single personal attack: They're dealing with a sadistic rapist turned killer who is terrorizing rural Grant County.
Jeffrey isn't alone in his search. Lena Adams - the county's sole female detective - wants to see justice done since her sister was the first victim. Sara, too, cannot escape the terror. A secret from her past could hold the key to finding the killer - unless he finds her first.