Crosshairs by James Patterson

"Turn. Up. The. Heat" (New York Times) with this psychological thriller. New York City detective Michael Bennett faces his most terrifying killer ever. It could be anyone. They could be anywhere.
A killer uses fearsome precision to take out impossible targets.
Detective Michael Bennett teams with a shooting expert-a former Army Ranger and sniper with NYPD's Emergency Services Unit. But Officer Rob Trilling seems more comfortable with rifles than he is with people.
When his new partner begins to log unexplained absences from duty, only Bennett can prove whether the decorated officer is a lonely hunter or a hardened assassin.