In the early 2000s, a top member of the Sinaloa drug cartel said he intercepted a rival organization’s shipment of cocaine as it passed through the Mexican state of Chiapas.
After the drugs had been taken to a warehouse, the member testified on Monday, one of the cartel’s bosses showed up with an unexpected guest: Genaro García Luna, who was then in charge of Mexico’s equivalent of the F.B.I.
An agreement had been reached, according to the cartel member, Sergio Villarreal Barragán. The cartel would take half of the profits from the sale of the two-ton load. Mr. García Luna, he said, would get the other half — or more than $14 million.
The account of the warehouse deal came on the first day of Mr. García Luna’s corruption trial in Federal District Court in Brooklyn. In dueling opening statements, the government and the defense offered radically different descriptions of the defendant.
Prosecutors said that for more than a decade, Mr. García Luna led a double life, taking millions of dollars in bribes to protect the very traffickers he was supposed to be pursuing.
But his lawyers countered these arguments. They said Mr. García Luna, who once ran Mexico’s version of the F.B.I., was in fact what he has always claimed to be: an honest lawman who had helped the United States arrest top figures in the Sinaloa drug cartel. Those same criminals, the lawyers said, had now returned to seek revenge against him as government witnesses.
The trial, which could last as long as eight weeks, will present the jury with a stark choice: Was Mr. García Luna, the highest-ranking Mexican official ever to stand trial in a U.S. courtroom on drug trafficking charges, a scourge of the Sinaloa cartel or a secret servant to it?
Along the way, the proceeding will take the jurors on a tour of the dizzying hall of mirrors that often exists inside the corridors of power in Mexico.
Philip Pilmar, a federal prosecutor, opened the government’s case by laying out the defendant’s professional biography.
He told the jury that Mr. García Luna entered public service in 1989 by working for C.I.S.E.N., a newly formed intelligence agency in Mexico. From 2001 to 2006, he served as director of the Federal Investigative Agency. Then, under former President Felipe Calderon, he was named Mexico’s public security secretary, a powerful cabinet-level position he held until 2012.
But all that time, Mr. García Luna, who is accused of being part of a continuing criminal enterprise, was betraying his colleagues and his country, Mr. Pilmar said.
“While entrusted to work for the Mexican people, he also had a second job, a dirtier job, a more profitable job,” Mr. Pilmar said. That job, he went on, was protecting the Sinaloa cartel’s vast shipments of cocaine and other drugs as they crossed the border to American consumers.
In his own opening statement, César de Castro, Mr. García Luna’s lead lawyer, told the jury that, despite its claims, the government had no definitive evidence of his client’s guilt and that the prosecution’s case would rely almost exclusively on witnesses from within the cartel itself. Many of those witnesses were men who Mr. García Luna had helped to arrest in Mexico and extradite to the United States, providing them a motive to testify against him.
“What better revenge,” Mr. de Castro said, “than to bury the man who led the war against the cartels.”
Mr. de Castro also pointed out that throughout his long career, Mr. García Luna worked closely with a who’s who of top U.S. officials in the State and Justice Departments, as well as in Congress and the White House.
To that end, he showed the jury an array of photos of his client posing with Eric Holder, a former attorney general, and Hillary Clinton, the onetime secretary of state; and shaking hands with President Barack Obama.
After the opening statements, the government called Mr. Villarreal Barragán, a former police officer who switched sides in the drug war and went to work for the Sinaloa cartel in about 2001.
A towering man known as “El Grande,” Mr. Villarreal Barragán told the jury that he was present when his boss, a top cartel leader named Arturo Beltrán Leyva, gave bribes to Mr. García Luna — at one point, on a monthly basis.
While he never cited a specific an amount, Mr. Villarreal Barragán said the money given to Mr. García Luna helped the cartel’s traffickers expand their operations from their home state, Sinaloa, to vast swaths of the rest of Mexico.
“The payments grew as the cartel grew,” Mr. Villarreal Barragán said, “and without that support it would have been practically impossible.”