The letter triggered a mini-crisis last month with Spain, the country that ruled Mexico as a colony for 300 years after its conquistadors arrived here and brought down the indigenous Aztec empire.
Lopez Obrador has lashed out repeatedly at the Reforma newspaper since it published the letter, calling on it to reveal its source “in the name of transparency.”
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He hinted in a press conference that he suspected the Spanish government may have leaked the document itself.
“It would be interesting to know if the Spanish government leaked the letter, because the connotation would be different,” he said.
“What moral authority can a government have that leaks documents?”
Spain has said it “deeply regrets” the letter was made public, and that the matter should have been confined to diplomatic channels.
It flatly rejected Lopez Obrador's demand for an apology, saying the conquest “cannot be judged in the light of contemporary considerations.”
Lopez Obrador owned up to the letter after it was made public, saying that both King Felipe VI and Pope Francis should apologize for the “massacres and oppression” committed in the name of colonizing and evangelizing the indigenous peoples of what is now Mexico.
Free-press advocates have criticized Lopez Obrador, an anti-establishment leftist who took office in December, for insisting the newspaper reveal its source.
“Calling on (journalists) to reveal their sources is worrying, because it can inhibit people from giving information to the press,” said the head of media rights group Article 19 in Mexico, Ana Cristina Ruelas.
“It speaks to a lack of recognition for the importance of a free press in a democratic society,” she told Reforma.
“The experts can calm down, nothing is going to happen” if Reforma refuses to reveal its source, Lopez Obrador said.
Mexico City, where Reforma is based, has a law protecting the right of journalists not to reveal their sources.