Still See You Everywhere by Lisa Gardner

An instant New York Times bestseller! 

My name is Frankie Elkin, and finding missing people is what I do.
 
From #1 New York Times bestselling author Lisa Gardner comes a harrowing locked-room style thriller set on a remote island. Frankie Elkin is an expert at finding missing people, but not even she could have anticipated this latest request—to locate the long-lost sister of a female serial killer facing execution in three weeks’ time.

"The very definition of the word unputdownable." -- Tess Gerritsen
 
Frankie Elkin is an expert at finding the missing persons that the rest of the world has forgotten, but even she couldn’t have anticipated this latest request—to locate the long-lost sister of a female serial killer facing execution in three weeks’ time.