Killer Heat by Brenda Novak

The bodies of seven women have been discovered in Skull Valley, Arizona. Jonah Young- a private security operative from Department 6- has been hired by the Yavapai County sheriff to assist in solving these murders. But Jonah's not prepared for the complications that arise when he's forced to work with a woman from his past- private investigator Francesca Moretti.
Jonah betrayed Francesca ten years ago. She hasn't forgiven him and she's pretty sure she never will. But the woman she was hired to find has been murdered in exactly the same way as the seven in Skull Valley, so Francesca has to work closely with Jonah. They quickly zero in on the most likely suspect- but questions still remain. Questions they have to answer. Because if they bet on the wrong man, it might be the last thing they ever do