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It says a lot about the El Chapo trial that the biggest news to emerge from court last week was about something that didn’t happen.

A top member of the Sinaloa drug cartel was expected to take the witness stand and testify that he had paid at least $6 million in bribes to one of Mexico’s presidents. Which one? Nobody really knew. But the witness never got the chance to tell his story after prosecutors objected to the line of questioning and a federal judge, largely working in secret, barred the testimony from being heard.

The development — or rather, the lack of one — reflects what is likely to be an ongoing tension at the trial, which enters its third week on Monday in Federal District Court in Brooklyn. The prosecution hopes to keep the evidence narrowly focused on El Chapo, whose real name is Joaquín Guzmán Loera, and his violent reign as one of the world’s biggest drug dealers. The defense, to deflect attention away from its client, is trying to present proof about systemic corruption in the Mexican government.

Some tales of graft did come out last week as the star witness from the cartel, Jesus Zambada García, admitted to bribing one of Mexico’s highest-ranking law enforcement officials, Genero García Luna, in a stunning bit of testimony that sent a group of Mexican reporters covering the trial dashing from the courtroom to alert their editors. Mr. Zambada also confessed that he once paid $250,000 to a Mexican army officer to abort a military operation to capture Mr. Guzmán.