As years passed, Mr. Zambada said, he rose through the ranks and became his father’s top lieutenant, overseeing cocaine shipments from Colombia to Mexico, and from Mexico across the United States border to cities like Chicago and Los Angeles. At different times, he played numerous and varied roles for the cartel: ambassador, operations manager and messenger boy.
In 2009, however, Mr. Zambada was arrested during an Army operation in Mexico City and extradited to Chicago. At first, he was expected to be prosecuted there on charges of smuggling tons of drugs while serving as his father’s right-hand man. But before his trial began, his lawyers dropped a bombshell: They claimed that for years he had been working secretly as a spy for the D.E.A., swapping information about his rivals in exchange for the ability to run his business freely.
While American authorities have acknowledged that Mr. Zambada met with federal agents, they have long denied there was any quid pro quo agreement. In a recent ruling, Judge Brian M. Cogan, who is hearing Mr. Guzmán’s case, said that Mr. Zambada’s claims regarding his cooperation with the Americans cannot be mentioned at the trial.
Mr. Zambada ultimately pleaded guilty to drug trafficking charges in a secret proceeding in Chicago in 2013. And for the last five years, he has been waiting to appear in court and tell his story.
He did so on Thursday, energetically telling jurors how he once planned to break Mr. Guzmán’s brother out of prison with a helicopter. (The brother, Arturo, was killed before the escape could be attempted.) He also testified that in 2007 he met with a group of “high-level politicians” and representatives from Pemex, Mexico’s national oil company, to discuss a scheme to ship 100 tons of cocaine in a tanker vessel owned by the firm.
But the focus of his testimony on Thursday was his father.
One of the first questions the prosecution asked him was: “What does your dad do for a living?”
There was only one answer.
“My dad is the Sinaloa cartel’s leader,” Mr. Zambada said.