A Twist of the Knife by Becky Masterman

A former colleague of retired FBI agent Brigid Quinn asks her to come to Florida and assist on an appeal for Marcus Creighton, who's soon to be executed for the murders of his family, although his children's bodies were never found. Brigid agrees to help her friend, Laura Coleman with the case. A native Floridian, Brigid will also be able to spend time with her elderly father, who's in the hospital and not doing well. Brigid and Laura have an uphill battle to save Creighton's life by searching for old evidence and seeing if any mistakes were made with the original case. Laura, though, appears to be a little too close to Creighton for Brigid's liking. In addition to all this, they are also trying to figure out what really happened to the Creighton children all those years ago.