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.”