MEXICO CITY—When Jimmy Morales ran for president of Guatemala three years ago, the former TV comedian’s campaign slogan was “Neither corrupt, nor a thief.”

One of the reasons Morales had to make honesty a central issue in his campaign was the work of an unusually powerful group of crime fighters known as the International Commission Against Impunity in Guatemala, or CICIG.

In the face of enormous political pressure, this United Nations-backed organization had conducted rigorous investigations of corruption at the highest levels of Guatemala’s government for more than a decade. Its work helped inspire a nationwide protest movement in 2015 and led to the arrest of then-President Otto Pérez Molina, then-Vice President Roxana Baldetti and many more.

For more than a year, CICIG has been investigating Morales himself, accusing him of accepting about $1 million (U.S.) in illegal campaign donations and earning his ire in return.

Last year, Morales tried to expel the head of CICIG, Colombian prosecutor Iván Velásquez, but the Supreme Court blocked the move.

Over the past week, the conflict has flared up again. On Aug. 31, Morales said he would not renew CICIG’s mandate, which expires next year. The same day, Guatemalan military vehicles stood guard outside CICIG’s offices and descended on a central plaza. On Tuesday, Morales ordered that Velásquez, who has led CICIG since 2013, not be allowed back in Guatemala.

“For some time now, there have been efforts to derail anti-corruption efforts in Guatemala and continued attacks against the commission and the commissioner,” said Adriana Beltrán, a Guatemala expert at the Washington Office on Latin America. Morales’s actions, she said, are “his attempt to protect himself, given the continuing probe against him.”

While Velásquez remains in the United States, the work of CICIG continues, said a spokesman, Matias Ponce. The organization, which has about 200 staff members, is also waiting for the Guatemalan government to renew work visas of CICIG’s foreign staff, he said.

Apart from blocking Velásquez’s entrance into Guatemala, the Morales government this year removed 25 police personnel assigned to guard CICIG, cutting its security force in half.

Morales has argued that CICIG, as a foreign body that receives U.S. funding, constitutes a violation of Guatemalan sovereignty and that Guatemala’s own judicial institutions should be handling such graft cases.

CICIG works in conjunction with the Guatemalan attorney general’s office in building corruption cases.

In a letter to UN Secretary General António Guterres last week, Morales said CICIG has had more than “sufficient” time over the course of its mandate to achieve its goals.

CICIG was set up in 2006 to bolster Guatemala’s weak judicial institutions. At the time, impunity was rampant in the country and murders were hardly ever solved. The group, composed of investigators from around the world, used sophisticated investigative techniques, wiretapping and examination of financial records to pursue high-profile crimes. Its work became a model and inspiration in Latin America, where corruption often goes unpunished.

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But CICIG has also been polarizing. Critics see it as overzealous and manipulated for political reasons. Earlier this year, Sen. Marco Rubio, R-Fla., put on hold $6 million in State Department funding to CICIG, saying he was concerned that Russia had “manipulated” the group into pushing for the prosecution of a Russian family in Guatemala.

CICIG’s investigation against Morales had also been gaining steam. Last month, Velásquez, along with Guatemalan Attorney General María Consuelo Porras, asked the nation’s Congress to strip Morales of his immunity from prosecution. A congressional commission has been formed to weigh the request.