“Mr. Guzmán paid a bribe of $100 million to President Peña Nieto?” Jeffrey Lichtman, one of Mr. Guzmán’s lawyers asked Mr. Cifuentes during cross-examination.

“Yes,” responded Mr. Cifuentes.

The bribe was delivered to Mr. Peña Nieto through an intermediary, according to Mr. Cifuentes.

While other witnesses at Mr. Guzmán’s trial in Federal District Court in Brooklyn have testified about payoffs from traffickers to the Mexican police and public officials, the revelations about Mr. Peña Nieto were the most egregious allegations yet. If true, they suggest that corruption by drug cartels had reached into the highest level of Mexico’s political establishment.