THE BOY SITS IN A RUN-DOWN MOTEL ROOM IN Laredo, Texas, waiting for the phone to ring. His name is Gabriel Cardona, and he is 18 years old, handsome, and serious. His lips are full, and his dark, brooding eyes give him the sad look of an old Catholic saint, the type that line the walls of his motherís home in fading lithographs. When the phone rings, the boy answers it. He listens and hangs up. He gets in his red Jetta and drives to an abandoned house to pick up the man he was directed to meet. Together they head out of town, toward the warehouse district.

It is the summer of 2005, and the Mexican drug cartels are at war over the lucrative I-35 corridor, which stretches from the border town of Laredo north to Dallas and on to Chicago. Day after day, members of rival cartels kill one another for control of the entrance to the highway and the narcotics that flow along it. Day after day, police are murdered. In Nuevo Laredo, the sister city just south of the border, people profit or perish at an astonishing clip. Plata o ploma, as the Mexican saying goes: silver or lead. Bodies turn up in 55-gallon drums, or on top of smoldering tires, charred to a crisp. Severed heads are found in the desert, cooling in iceboxes. Ranchers stumble across decapitated bodies in the brush, and their dogs find hacked-off legs wrapped in black garbage bags. Nobody on the U.S. side seems to care.

It has been this way for as long as the kid can remember. On the outskirts of town, Cardona and his new associate meet up with some of Cardona's friends. They stop in an industrial park, where they watch as a man leaves an office building. Bruno Orozco, a former Mexican cop who is now tied to Nuevo Laredo's criminal underworld, has been under surveillance for days. Cardona knows where he lives, where he works, where he stops for lunch. When Orozco gets into his Nissan Altima and begins to drive, Cardona, his friends, and the associate follow. The associate flips on a set of fake police lights and Orozco pulls over. The associate asks Orozco to step out of his vehicle. As the man slaps a cuff on Orozco's wrist, Orozco realizes that something isnít right and begins to struggle. He calls for help, but it's too late. Cardona's gang tosses Orozco onto his back and one of them unloads a clip from an AR-15 into his body. As Orozco thrashes in the grass, gasping and dying, a horrific new era of the drug war is born: Not only has a Mexican cartel brazenly staged a hit on the U.S. side of the border, but for the first time a cartel has used a squad of American teenagers to do its killing.

Up close four years later, Cardona, now 22 years old, looks more like a child than an assassin. His wrists are thin, his shoulders barely fill out his prison whites, and with his smooth, unblemished face—and those sad, saintly eyes—he has the delicate look of an altar boy. Like his mother, he was named after the angel Gabriel, the biblical messenger who foretold the birth of a son of God who would die to cleanse the world of sin. Blood and sin. This much of his Catholic religion Cardona understands. For $500 a week, he and his team of teenage hit men would hang in abandoned safe houses or motel rooms, smoking pot, playing Xbox, waiting for the call to come—the order for a murder that could pay as much as $10,000 and two kilos of coke. "Along the border, this is all there is," Cardona tells me. "Youíre either a cop, a federal agent, or a drug trafficker. For kids like me, there's only one path."