In the struggle to defeat transnational crime in Central America, the U.S. is financing a United Nations prosecutorial body in Guatemala. Yet these U.N. prosecutors are thumbing their noses at the rule of law and seem to be using their power to politicize the Guatemalan judiciary.

This is dividing and destabilizing a pivotal democracy in the region. The fragile Guatemalan state is in the crosshairs of Venezuelan dictator Nicolás Maduro and Cuba’s Gen. Raúl Castro. If their allies seize control of Mexico’s southern neighbor via its institutions, as Daniel Ortega has done in Nicaragua, it will have implications for Mexican and American security.

The U.N. body, known as the International Commission Against Impunity in Guatemala (CICIG by its Spanish initials), has been in the country since 2007. It has busted some criminals. But its unchecked power has led to abuse, and this should concern U.S. backers. Some of CICIG’s most vociferous defenders hail from Guatemala’s extreme left, which eschews equality under the law and representative democracy.

CICIG’s rogue justice has come to the attention of Sen. Roger Wicker (R., Miss.), chairman of the Commission on Security and Cooperation in Europe, also known as the Helsinki Commission. He has scheduled a hearing April 27 to review CICIG’s role in the Guatemalan prosecution and extralegal conviction of a Russian family on the run from Vladimir Putin’s mafia.

As I detailed in March 26 and April 19 Americas columns, Igor and Irina Bitkov, and their daughter Anastasia, fled persecution in Russia and became victims of a crime syndicate in Guatemala that was selling false identity documents. Yet Guatemala and CICIG tried the family alongside members of the crime ring that tricked them. They were convicted and given unusually harsh sentences.

Guatemalan law and the U.N.’s Palermo Convention say that such migrants are victims, and a Guatemalan constitutional appeals court ruled that the Bitkovs committed no crime. CICIG and Guatemalan prosecutors ignored that ruling, went to a lower court and got a conviction. CICIG will not say why, or why it didn’t prosecute the law firm that solicited the fake documents given to the Bitkovs.

Matías Ponce is “head of communications” for CICIG but there is no contact information for him or his office on the CICIG website. I managed to get his cellphone number from a third party and, after repeated tries, made contact with him. I requested his email and wrote to him so I could share with readers CICIG’s explanation of what appears to be abuse of power. He sent me a boilerplate response about CICIG’s work against criminal networks but no answers to my questions.

It is unlikely CICIG will answer questions before the Helsinki Commission. Its co-chairman, Rep. Chris Smith (R., N.J.), invited CICIG to appear at a similar hearing he proposed for April 24 in the House Foreign Affairs subcommittee monitoring human rights and U.N. entities. CICIG declined the invitation. That hearing was not scheduled, though the office of Foreign Affairs Chairman Ed Royce (R., Calif.) told me it’s not dead. If CICIG refuses to cooperate with the Helsinki Commission, it will fuel the feeling among rule-of-law advocates that it has something to hide.

CICIG says it is in Guatemala merely to “support” the attorney general in her work “identifying and dismantling” criminal networks and is not involved in politics. But an academic analysis of CICIG by Jonatán Lemus, a Francisco Marroquín University political science professor, suggests otherwise. Mr. Lemus observes that “CICIG has also been criticized for the very same reasons others have praised it: becoming a player in judicial appointments, proposing some controversial reforms to the Guatemalan constitution, and the use of televised conferences to shift the public in its favor. From this perspective, instead of strengthening Guatemalan institutions, the Commission is making national institutions dependent on its assistance.”

This dependence drives CICIG deeper into politics. As Mr. Lemus notes, “once immersed in a polarized political system,” an international body designed like CICIG naturally “will face incentives to behave as any domestic bureaucracy trying to maximize its power and resources to ensure its survival.”

Without an explanation for the bizarre Bitkov convictions, Guatemalans are left to speculate about CICIG’s motives. Incompetence is one possibility. But once the injustice was publicized and not corrected, that reasoning collapsed. A foreign businessman also makes an easy target for a politically correct prosecutor seeking approval from anticapitalist nongovernmental organizations.

Kremlin “influence” cannot be ruled out. Nailing the Bitkovs was a priority for Russia because the family had refused to “donate” large sums to the Putin kitty in Kaliningrad. It would hardly be surprising to learn that Moscow leaned on prosecutors and judges to put the family behind bars.

There’s no doubt that something fishy went on, and CICIG prosecutor Iván Velásquez’s unwillingness to address it is troubling. The truth matters for the family, for Guatemala and for the U.S.