Another former drug agency official offered this explanation for the laundering operations: “Building up the evidence to connect the cash to drugs, and connect the first cash pickup to a cartel’s command and control, is a very time consuming process. These people aren’t running a drugstore in downtown L.A. that we can go and lock the doors and place a seizure sticker on the window. These are sophisticated, international operations that practice very tight security. And as far as the Mexican cartels go, they operate in a corrupt country, from cities that the cops can’t even go into.”

The laundering operations that the United States conducts elsewhere — about 50 so-called Attorney General Exempt Operations are under way around the world — had been forbidden in Mexico after American customs agents conducted a cross-border sting without notifying Mexican authorities in 1998, which was how most American undercover work was conducted there up to that point.

But that changed in recent years after President Felipe Calderón declared war against the country’s drug cartels and enlisted the United States to play a leading role in fighting them because of concerns that his security forces had little experience and long histories of corruption.

Today, in operations supervised by the Justice Department and orchestrated to get around sovereignty restrictions, the United States is running numerous undercover laundering investigations against Mexico’s most powerful cartels. One D.E.A. official said it was not unusual for American agents to pick up two or three loads of Mexican drug money each week. A second official said that as Mexican cartels extended their operations from Latin America to Africa, Europe and the Middle East, the reach of the operations had grown as well. When asked how much money had been laundered as a part of the operations, the official would only say, “A lot.”

“If you’re going to get into the business of laundering money,” the official added, “then you have to be able to launder money.”

Former counternarcotics officials, who also would speak only on the condition of anonymity about clandestine operations, offered a clearer glimpse of their scale and how they worked. In some cases, the officials said, Mexican agents, posing as smugglers and accompanied by American authorities, pick up traffickers’ cash in Mexico. American agents transport the cash on government flights to the United States, where it is deposited into traffickers’ accounts, and then wired to companies that provide goods and services to the cartel.

In other cases, D.E.A. agents, posing as launderers, pick up drug proceeds in the United States, deposit them in banks in this country and then wire them to the traffickers in Mexico.