In August of last year, a window opened into this shadowy world: A tip led federal drug agents to Vargas, a low-level courier willing to tell them what he knew in exchange for leniency.

“Sometimes,” one agent later explained, “the dope gods smile.”

Vargas was the perfect drug mule. He was 22 but looked younger. He’d been born in California, moving to Mexico at age 12 after his father was deported, so he possessed a U.S. passport. He also had a spotless record, perfect English and a desperate need for cash: His father had already lost one eye to diabetes.

He’d been offered $6 a gram. This job would earn him nearly $6,000.

Things could go wrong. Another courier headed to Dayton had to use the bathroom unexpectedly during a layover at a U.S. airport and lost his pellets when the toilet flushed automatically, according to the drug agents who finally arrested him.

Pellets bursting was a courier’s worst fear. Once in Lorain, Ohio, a courier started foaming at the mouth, and his handler called down to Mexico to figure out what to do. As authorities listened via wiretap, the handler was told to cut the courier open and retrieve the remaining drugs.

Vargas had been carefully trained to avoid such accidents. According to court documents and multiple interviews with Drug Enforcement Administration investigators and Vargas’s attorneys, he began his journeys with visits to a gray stucco house in Uruapan, Mexico, a city of 315,000 people in the state of Michoacan, which sprawls west from Mexico City to the Pacific Ocean.

But Vargas knew almost nothing about Dayton, beyond what seemed to be an insatiable demand for the secret stash he carried.