As the United States has opened new law enforcement and intelligence outposts across Mexico in recent years, Washington’s networks of informants have grown there as well, current and former officials said. They have helped Mexican authorities capture or kill about two dozen high-ranking and midlevel drug traffickers, and sometimes have given American counternarcotics agents access to the top leaders of the cartels they are trying to dismantle.

Typically, the officials said, Mexico is kept in the dark about the United States’ contacts with its most secret informants — including Mexican law enforcement officers, elected officials and cartel operatives — partly because of concerns about corruption among the Mexican police, and partly because of laws prohibiting American security forces from operating on Mexican soil. …

As important as informants have been, complicated ethical issues tend to arise when law enforcement officers make deals with criminals. Few informants, law enforcement officials say, decide to start providing information to the government out of altruism; typically, they are caught committing a crime and want to mitigate their legal troubles, or are essentially taking bribes to inform on their colleagues. …

Vanda Felbab-Brown, an expert on organized crime at the Brookings Institution, said that while some had criticized the D.E.A. for entertaining “deals with the devil,” she saw the Zambada case as an important intelligence coup. Even in an age of high-tech surveillance, she said, there is no substitute for human sources’ feeding authorities everything from what targeted traffickers like to eat to where they sleep most nights.