MEXICO CITY — At first, it looked as if another dirty politician had been nabbed.

Humberto Moreira, a former Mexican governor, was detained in 2016 as he was about to board a plane in Madrid. The allegations — including money-laundering — were big news back home. But they were hardly a shock. Moreira’s administration had been accused of corruption relating to drug cartel violence and the hundredfold increase in government debt during his time in office from 2006 to 2011.

“Moreira is a politician who emits a stench of corruption,” Sergio Aguayo, one of Mexico’s best-known academics, wrote in a column published by the national newspaper Reforma five days after the arrest.

Today, Aguayo is the one facing judicial action: A $530,000 fine for slandering the former governor, who was released from a Spanish prison for lack of evidence.

The case has provoked an outcry in Mexico about freedom of expression and the silencing of those who criticize the powerful.

Press-freedom advocates worry this young democracy is sliding backward. A draft of an expected new judicial reform package obtained by journalists this month included a proposal that would make defamation a criminal offense.

Aguayo’s case has crystallized the potential implications of such a measure. Though it’s been denounced by Mexico City’s Mayor, Claudia Sheinbaum, among others, it’s sent a chill through newsrooms and editorial offices.

The sentence “sets a dangerous precedent in Mexico, where journalists are increasingly targeted by powerful politicians in multimillion-peso lawsuits aimed at silencing critical voices,” said Jan-Albert Hootsen, Mexico representative for the Committee to Protect Journalists.

President Andrés Manuel López Obrador was confronted on Wednesday at his daily news conference by Denise Dresser, a political scientist and columnist at Reforma. She asked if the president would promise not to support a new law criminalizing defamation.

He agreed, saying: “We have a commitment to guarantee freedom of expression, the exchange of ideas, the right to dissent.” López Obrador, a leftist nationalist, has frequently been criticized himself for bashing journalists who publish news he dislikes.

Moreira’s term as governor was marked by gruesome violence, as the Zetas drug cartel carried out some of Mexico’s worst massacres of recent years.

In three separate trials of Zetas members later carried out in U.S. federal courts, “multiple witnesses described bribery payments of millions of dollars to Humberto Moreira” and his brother Ruben, who succeeded him as governor, “in exchange for complete control of the state,” researchers at the University of Texas law school reported in 2017. The allegations were also included in reports by Mexican human rights groups.

Moreira and his brother have denied such reports and any ties with the Zetas. Humberto Moreira has said the accounts are suspect because they came from drug traffickers seeking to reduce their U.S. sentences. There was no answer Thursday at the office of Moreira’s lawyer, Ulrich Richter.

After leaving the governorship, Moreira served for several months as head of Mexico’s long-ruling Institutional Revolutionary Party. He stepped down in December 2011 amid revelations that his state’s debt had ballooned to $2.5 billion under his rule.

Aguayo, a human-rights activist and professor at the prestigious Colegio de Mexico, published a book in March 2016 documenting the abuses carried out by the Zetas during Moreira’s term.

“I think this was the motive” for the lawsuit, Aguayo told The Washington Post on Thursday, noting it wasn’t filed until five months after his column appeared. The professor, now retired and living on an income of around $30,000 a year, has continued to investigate the violence of that era.

Last March, a court found Aguayo not guilty of the charge of causing “moral damage” to Moreira. Then, in October, another judge in Mexico City reversed the decision. The professor appealed. But this week, the court ordered him to hand over a guarantee of about $24,000 toward the half-million-dollar fine — or face the seizure of his home. Aguayo complied.

His legal team has charged that the lead judge on the case has a conflict of interest: They say the magistrate’s brother was awarded a lucrative license to open a notary business by Ruben Moreira.

“I fell into the hands of judges who acted in a biased way against me,” Aguayo said. “I don’t know whether it was due to corruption, or owing favors, or other motives.”

In the latest twist in the legal saga, Mexico’s Supreme Court indicated on Wednesday that it might assume control over the case. Lawmakers in the Mexico City congress have also called for an investigation of the judges involved in the case.