The agents, primarily with the Drug Enforcement Administration, have handled shipments of hundreds of thousands of dollars in illegal cash across borders, those officials said, to identify how criminal organizations move their money, where they keep their assets and, most important, who their leaders are.

They said agents had deposited the drug proceeds in accounts designated by traffickers, or in shell accounts set up by agents.

The officials said that while the D.E.A. conducted such operations in other countries, it began doing so in Mexico only in the past few years. The high-risk activities raise delicate questions about the agency’s effectiveness in bringing down drug kingpins, underscore diplomatic concerns about Mexican sovereignty, and blur the line between surveillance and facilitating crime. As it launders drug money, the agency often allows cartels to continue their operations over months or even years before making seizures or arrests…

Another former agency official, who asked not to be identified speaking publicly about delicate operations, said, “My rule was that if we are going to launder money, we better show results. Otherwise, the D.E.A. could wind up being the largest money launderer in the business, and that money results in violence and deaths.”