By Ginger Thompson | NYTimes.com

Undercover American narcotics agents have laundered or smuggled millions of dollars in drug proceeds as part of Washington’s expanding role in Mexico’s fight against drug cartels, according to current and former federal law enforcement officials.

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.

A crime scene in Monterrey, Mexico, last week. Drug-related violence has claimed the lives of more than 40,000 people since late 2006, Mexican officials say. Josue Gonzalez/Reuters