Mexico's transparency and data protection office announced Thursday a probe of President Andres Manuel Lopez Obrador's disclosure of a journalist's telephone number at his daily press briefing.
Lopez Obrador revealed the number of The New York Times reporter while reading out questions from the newspaper related to a report it was about to publish about alleged drug cartel links to his allies.
The National Institute for Transparency, Access to Information and Personal Data Protection said that it would investigate whether the disclosure broke the law.
The controversy comes weeks after a leak of journalists' personal data held by the Mexican government triggered alarm among media rights activists in one of the world's most dangerous countries for the press.
The New York Times described the incident as "a troubling and unacceptable tactic from a world leader at a time when threats against journalists are on the rise."
On Thursday, the newspaper reported that US law enforcement officials spent years looking at allegations that people close to Lopez Obrador took millions of dollars from criminal gangs.
The United States decided not to open a formal investigation because there was "little appetite to pursue allegations against the leader of one of America's top allies," the article said.
While US officials identified possible links between drug cartels and people close to Lopez Obrador, they did not find any direct ties between the president himself and criminal groups, it said, citing US records and unidentified sources familiar with the matter.
The information was hard to corroborate because much of it came from informants, according to the newspaper.
Lopez Obrador described the accusations as "slander" and urged the US government to respond.
"If they don't want to say anything, if they do not want to act with transparency, that's their business, but any democratic government, defender of freedoms, would have to inform" people about the claims, he said.
It is the second time this year that Lopez Obrador, who took office in 2018, has faced such claims in the US media.
Last month, he rejected allegations in an article published on the ProPublica news site that drug traffickers helped fund his first presidential campaign in 2006.
The report said that it was unclear whether Lopez Obrador -- who narrowly lost the race -- sanctioned or was even aware of the funding.
He described those allegations as "completely false" and portrayed the claims as a political attack by his opponents ahead of presidential elections in June, which he hopes to see his close ally Claudia Sheinbaum win.
