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WASHINGTON — American law enforcement agencies have significantly built up networks of Mexican informants that have allowed them to secretly infiltrate some of that country’s most powerful and dangerous criminal organizations, according to security officials on both sides of the border.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.
“The Mexicans sort of roll their eyes and say we know it’s happening, even though it’s not supposed to be happening,” said Eric L. Olson, an expert on Mexican security matters at the Woodrow Wilson Center.
“That’s what makes this so hard,” he said. “The United States is using tools in a country where officials are still uncomfortable with those tools.”
In recent years, Mexican attitudes about American involvement in matters of national security have softened, as waves of drug-related violence have left about 40,000 people dead. And the United States, hoping to shore up Mexico’s stability and prevent its violence from spilling across the border, has expanded its role in ways unthinkable five years ago, including flying drones in Mexican skies.
The efforts have been credited with breaking up several of Mexico’s largest cartels into smaller — and presumably less dangerous — crime groups. But the violence continues, as does the northward flow of illegal drugs.
While using informants remains a largely clandestine affair, several recent cases have shed light on the kinds of investigations they have helped crack, including a plot this month in which the United States accused an Iranian-American car salesman of trying to hire killers from a Mexican drug cartel, known as Los Zetas, to assassinate the Saudi ambassador to Washington.
American officials said Drug Enforcement Administration informants with links to the cartels helped the authorities to track down several suspects linked to the February murder of a United States Immigration and Customs Enforcement agent, Jaime J. Zapata, who is alleged to have been shot to death by members of Los Zetas in central Mexico.
The D.E.A.’s dealings with informants and drug traffickers — sometimes, officials acknowledged, they are one and the same — are at the center of proceedings in a federal courthouse in Chicago, where one of the highest-ranking leaders of the Sinaloa cartel is scheduled to go on trial next year.
And last month, a federal judge in El Paso sentenced a midlevel leader of the Sinaloa cartel to life in prison after he was found guilty on drug and conspiracy charges. He was accused of working as a kind of double agent, providing the Immigration and Customs Enforcement agency with information about the movements of a rival cartel in order to divert attention from his own trafficking activities.
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.
Morris Panner, a former assistant United States attorney who is a senior adviser at the Center for International Criminal Justice at Harvard Law School, said some of the recent cases involving informants highlight those issues and demonstrate that the threats posed by Mexican narcotics networks go far beyond the drug trade.
“Mexican organized crime groups have morphed from drug trafficking organizations into something new and far more dangerous,” Mr. Panner said. “The Zetas now are active in extortion, human trafficking, money laundering, and increasingly, anything a violent criminal organization can do to make money, whether in Mexico, Guatemala or, it appears, the U.S.”
Because of the clandestine nature of their communications with informants, and the potential for diplomatic flare-ups between the United States and Mexico, American officials were reluctant to provide any details about the scope of their confidential sources south of the border.
Over the past two years, officials said, D.E.A. agents in Houston managed to develop “several highly placed confidential sources with direct access” to important leaders of the Gulf Cartel and Los Zetas. This paid informant network is a centerpiece of the Houston office’s efforts to infiltrate the “command and control” ranks of the two groups.
One of those paid informants was the man who authorities say was approached last spring by a man charged in Iran’s alleged plot to assassinate the Saudi ambassador. Law enforcement documents say the informant told his handlers that an Iranian-American, Mansour J. Arbabsiar, had reached out to him to ask whether Los Zetas would be willing to carry out terrorist attacks in the United States and elsewhere.
Authorities would provide only vague details about the informant and his connections to Los Zetas, saying that he had been charged in the United States with narcotics crimes and that those charges had been dropped because he had “previously provided reliable and independently corroborated information to federal law enforcement agents” that “led to numerous seizures of narcotics.”
The Justice Department has been more forthcoming about the D.E.A.’s work with informants in a case against Jesús Vicente Zambada-Niebla, known as Vicentillo. Officials describe Mr. Zambada-Niebla as a logistics coordinator for the Sinaloa cartel, considered one of the world’s most important drug trafficking groups. His lawyers have argued that he was an informant for the Drug Enforcement Administration, which offered him immunity in exchange for his cooperation.
The D.E.A. has denied that allegation, and the Justice Department took the rare step of disclosing the agency’s contacts with him in court documents. The intermediary was Humberto Loya-Castro, who was both a confidant to the cartel’s kingpin, Joaquín Guzmán, known as El Chapo, and an informant to the D.E.A.
The documents do not say when the relationship between the agency and Mr. Loya-Castro began, but they indicate that because of his cooperation, the D.E.A. dismissed a 13-year-old conspiracy charge against him in 2008.
In 2009, the documents said, Mr. Loya-Castro arranged a meeting between two D.E.A. agents and Mr. Zambada-Niebla, who was floating an offer to negotiate some kind of cooperation agreement. But on the day of the meeting, the agents’ supervisors canceled it, expressing “concern about American agents meeting with a high-level cartel member like Zambada-Niebla.”
Mr. Zambada-Niebla and Mr. Loya-Castro showed up at the agents’ hotel anyway. The D.E.A. agents sent Mr. Zambada-Niebla away without making any promises, the documents said. A few hours later, Mr. Zambada-Niebla was captured by the Mexican police, and was extradited to the United States in February 2010.
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.
A former senior counternarcotics official echoed that thought.
“A D.E.A. agent’s job, first and foremost, is to get inside the body of those criminal organizations he or she is investigating,” the former official said, asking not to be identified because he occasionally does consulting work in Mexico. “Nothing provides that microscopic view more than a host that opens the door.”