One of the hundred books a year I read:

The titular “Mercy” refers to Atlee Pine’s twin sister who was abducted from their shared bedroom when both were small children. No body has ever been found and the crime remains unsolved, driving Atlee’s life as a loner.

A competitive weight lifter, she barely missed making the USA Olympic team but uses that strength to overpower bigger suspects in her way. She is assigned to a one-agent office in a small town, addressing her lone co-worker—an administrative assistant—by last name to keep her distance. With her straight shooting ways and job commitment, she gains the trust of locals including members of the Navajo nation and the park service rangers at the Grand Canyon, but considers no one a close friend.

She is called to assist in the disappearance of a tourist on a mule-trek to the floor of the canyon. Missing tourists are not uncommon, but finding a slaughtered mule points to a more dangerous element. Her initial investigation raises significant questions, including whether the missing man was who he claimed to be, and also attracts an unusual level of attention for such a minor case from high-placed government officials.

The attempts to dissuade Agent Pine have the opposite impact causing her to go to great lengths to solve the mystery, a case of international intrigue and deception.