Cleaning the Gold by Karin Slaughter, Lee Child

Twice the action.
Twice the drama.
Double the trouble.
Will Trent is undercover at Fort Knox. His assignment: to investigate a twenty-two-year-old murder. His suspect's name: Jack Reacher.
Jack Reacher is in Fort Knox on his own mission: to bring down a dangerous criminal ring operating at the heart of America's military. Except now Will Trent is on the scene.
But there's a bigger conspiracy at play - one that neither the special agent nor the ex-military cop could have anticipated. And the only option is for Jack Reacher and Will Trent to team up and play nicely. If they can...