Moore watched an R.V. turn onto the quiet street and nose into the warehouse’s garage door. The driver, Walter Ogden, the retiree from Oklahoma, got out, and Czach helped him load some duffel bags. A “routine” traffic stop after the R.V. drove away confirmed it: Ogden had picked up $1.96 million, just as Ramos said he would. Law-enforcement officials arrested Ogden — “that’s a tremendous amount of money to let walk,” Moore said — without revealing to the cartel that they now had an inside source.

Over the next six months, Moore and Ramos met twice a week in parking lots outside Walmart, Home Depot or Lowe’s. The organization generally worked like this, Ramos told him: Senior cartel leaders in Mexico would send the drugs to a house in Tucson, where a contact known as Viejo, the head of Detroit distribution, would hire a courier to drive the drugs to Ramos and other cartel members in Michigan. They would then sell it to Detroit’s biggest drug dealers, people like Pancho, the one-legged distributor. Pancho could have been the target of his own major D.E.A. investigation; but this case was so big that Pancho sat somewhere on the third tier of suspects. (His lawyer disputed that he was one of the largest drug dealers in Detroit.)

Ramos proved to be the ideal informant. While he was taking a tremendous risk in working with the D.E.A., he was perhaps less vulnerable than most. Authorities will not say where he is from, but he is not Mexican and, Graveline said, he may have felt somewhat less fearful because “his family is not down in Sinaloa country.”

With Moore listening in, Ramos would call cartel leaders in Mexico to discuss coming shipments. He agreed to wear video-recording devices into his meetings at Untouchables, an auto-body shop, and to the various parking lots where he met dealers in parked cars. It was Ramos who first told Moore about the elderly courier the cartel liked to work with. He only knew him as Tata.

On Sept. 17, 2011, Ramos met with Tata at Czach’s warehouse. It was Moore’s first sighting of Sharp. “I was kind of surprised that he seemed like he was in pretty good health,” Moore said. “When you hear 87 years old, you think of someone in a wheelchair. He was in good shape.”

While several men loaded up his truck with three duffel bags filled with cash, Tata cracked jokes about the drive and told the group that his doctor told him he would live to be 100. After the car was packed and Sharp was preparing to drive off, he asked Ramos to take some Georgia onions.

Onions? Moore had spent countless hours decoding the secret language of the cartel — cocaine was called food, heroin was called fea. Onions was a new one. Was it opium? Weapons?