Force of Nature (Joe Pickett 12) by C.J. Box
Used mass market paperback (cover may differ).
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He never wanted to tell Joe Pickett about it, but Nate Romanowski always knew trouble would eventually find him. Now, it has arrived, and he may not only have to fight for his life - but for Joe's too.
In 1995, Nate was in a Special Forces unit abroad, when a colleague committed a terrible act. Now a government big shot, this man is now determined to eliminate anyone who knows what he did. All too familiar with the ways of a mercenary, Nate knows exactly how he'll do it, too. In order to draw Nate out, he’ll strike at his friends - which means the entire Pickett family will be in grave danger.
Nate understands the game well, and he knows that there's only one way to fight back - and that's outside the law. Nate knows he can do it - but he's not so sure about straight-arrow Joe Pickett.