What Remains by Wendy Walker

Detective Elise Sutton is a forensics expert with a knack for solving cold cases and a deep knowledge of the criminal mind. She prides herself on being rational and in control, until a crisis at a department store leaves her steeped in guilt and self-doubt about whether she did the right thing to save a man's life.
Elise is hailed as a hero, but she doesn't feel like one. She soon grows numb, even to her husband and daughters, as she sets out to find the one man who might know the truth. When she finds him--or did he find her?--their connection sets off a terrifying game of cat and mouse, threatening Elise and the people she loves most.
Wendy Walker has crafted a brilliantly complicated, absorbing, and tension-filled psychological thriller with a shocking final twist that rivals The Woman in the Window and The Silent Patient.