WASHINGTON — 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...