Long Road to Mercy: David Baldacci

Long Road to Mercy is the heart-pounding first novel in the FBI Special Agent Atlee Pine series by bestselling author David Baldacci.
FBI Special Agent Atlee Pine has learnt three lessons in life:
Some wounds never heal.
Atlee’s twin sister, Mercy, was abducted from their bedroom over thirty years ago, and Atlee has spent every day since wondering what happened to her.
Time doesn’t lessen your pain.
The prime suspect, notorious serial killer Daniel James Tor, is in a high-security prison, but with no confession, Atlee continues to search for her sister, even as Tor taunts her from jail.
But she can always make a difference.
Wracked by survivor’s guilt, Atlee joined the FBI to hunt down killers like Tor. Assigned to the remote wilds of the Western United States, she has spent years honing her skills and building her endurance, always with one eye on the ultimate goal.
Now, Atlee Pine is tasked with an investigation which begins with a missing person in the Grand Canyon. And ends with a discovery much more sinister and far-reaching.