It was late November 2017, and Matthew Bowen, a veteran Border Patrol agent, was seething. A fellow Border Patrol agent in Texas had just been found dead in the field, and Bowen was certain someone who’d been crossing the border illegally was responsible for murdering him. “Snuffed out by some dirtbag,” Bowen, stationed in Nogales, Arizona, said in a text later obtained by federal authorities. Bowen, if lacking in evidence, wasn’t alone in his anger and suspicion.