Don't ever forget / Matthew Farrell.

Enhanced descriptions from Syndetics:

From the bestselling author of What Have You Done comes a mind-bending page-turner involving a series of dark crimes from the past.

When police investigator Susan Adler is called to the roadside murder of a fellow state trooper, she's tasked with finding the people responsible for the cold-blooded act caught on the trooper's dashboard cam. She traces the car to a nurse who, along with her elderly patient, has been missing for days. At the old man's house, she finds disturbing evidence that instantly revives two cold cases involving long-missing children.

The investigation takes a chilling turn when people involved with both the nurse and old man begin to turn up dead, and Susan enlists the help of her friend and forensic investigator Liam Dwyer. Together they must untangle the threads of this ever-more-complicated case--and stay ahead of whoever's trying to slash their progress. The old man's failing memory adds urgency: wherever he is, he's no doubt lost, confused, and in extreme danger.

What started as a traffic stop gone wrong quickly unfolds into one of the darkest cases of Susan's career, and it all leads to a sick, desperate killer. Susan and Liam must work fast to save the old man's life and keep future victims from the killer's grasp.