“If nothing more is presented,” he continued, “it would just be hearsay from a narcotrafficker hoping for some kind of benefit.”

María Celia Toro, a political analyst at the Colegio de México, a university in Mexico City, said that Mexicans have been conditioned by “a long history of false accusations.”

She added, “The most sensible thing to do is to wait.”

Many observers pointed out that if Mr. Guzmán had, indeed, paid Mr. Peña Nieto $100 million, then he did not get much bang for his buck, since he was eventually extradited to the United States by the president. Mr. Cifuentes dated the alleged bribe to October 2012 and Mr. Guzmán was captured for the first time in February 2014. He later escaped from prison only to be recaptured in January 2016 and extradited in January 2017.

“It isn’t logical that you would pay a bribe so that in a few years you will be captured,” said Juan Alberto Cedillo, a Mexican journalist who has investigated Mexican organized crime groups for more than a decade.

In addition, Mr. Peña Nieto might have surmised that any deals the two men had brokered would likely have arisen during judicial proceedings in the United States.

“For all his faults and flaws, Peña Nieto captured Chapo and extradited him, knowing full well that any information would filter out,” said Alejandro Hope, a security expert with a Mexican consulting firm, the Group of Economists and Associates. “The guy was being hunted down from Day 1 of Peña Nieto’s administration.”

Some who have questioned the veracity of Mr. Cifuentes’s testimony also pointed out that Mr. Peña Nieto is still in Mexico. He attended the funeral of a former governor of his home state several days ago, his first public appearance since leaving office. Were he guilty, some contended, he might have taken a page from the playbook of other politicians and fled, or gone into hiding.