CIUDAD JUÁREZ, Mexico — On a sun-scorched prison patio in this sprawling border town, a 23-year-old inmate calmly explained to me how he trafficked hundreds of guns a year from the United States to Mexico. He never bothered paying American citizen straw buyers to purchase the weapons for him, he said. Instead, he would go to one of the many weekend gun shows around Dallas and take advantage of the so-called gun show loophole to buy firearms from private sellers without a background check or proof of citizenship.

He would drive back to Mexico with about a dozen guns hidden in refrigerators and stoves in the back of his truck, and sell the weapons in his hometown, a few hours south of the Rio Grande. His most requested weapon, he told me, was the AR-15 semiautomatic rifle, which he could buy for as little as $500 and sell for five times that. He became richer than he had dreamed, buying a house and new trucks and motorbikes.

“At the beginning I felt bad, but you get used to it,” he said. “It’s the way you can have a good time. You sell weapons, you earn money and you have fun.” He got caught only because his cousin informed on him after an argument, he said, and he is now serving a nine-year sentence.

Guns from America inundate Mexico, arming the brutal cartels that have drowned this country in blood, destroyed families and driven people from their homes. Over a six-year period, the Justice Department traced more than 74,500 firearms seized from criminals here to the United States, where they were either manufactured or sold on after being imported from other countries.