The State Department has ended funding for the tracking of tens of thousands of Ukrainian children abducted by Russia, and American officials or contractors might have deleted a database with information on them, according to a letter U.S. lawmakers sent to Secretary of State Marco Rubio on Wednesday.

The work on the abducted children by the Yale School of Public Health Humanitarian Research Lab was frozen when President Trump signed an executive order in late January halting almost all foreign aid spending. Since then, Mr. Rubio and an official under him, Pete Marocco, have ended the vast majority of foreign aid contracts, including the one to the Yale lab.

The bipartisan congressional letter, signed by 17 lawmakers and organized by Representative Greg Landsman, Democrat of Ohio, said that “the foreign aid freeze has jeopardized, and may ultimately eliminate, our informational support of Ukraine on this front.”

The State Department and the Yale center “had been preserving evidence of abducted children from Ukraine it had identified, to be shared with Europol and the government of Ukraine to secure their return,” the letter said, according to a copy obtained by The New York Times. Europol is the main law enforcement agency of the European Union. The letter was also addressed to Scott Bessent, the Treasury secretary.

“We have reason to believe that the data from the repository has been permanently deleted,” it said. “If true, this would have devastating consequences. Can you please update us as to the status of the data from the evidence repository?”

A person familiar with the Yale center’s work said the details in the letter were accurate.

The Yale lab was one of several recipients of $26 million in congressional funding over three years through the State Department to track war crimes committed by Russian forces in Ukraine. That work began in 2022 under a program called the Conflict Observatory, which also documented atrocities in Sudan. Pages on the Conflict Observatory have been removed from the State Department website under Mr. Rubio, though its findings have been saved elsewhere online.

The department said in a statement after this article was published on Tuesday that it had ended the Yale lab’s award for work with the Conflict Observatory and that it did not hold data for the observatory. It referred questions to a nonprofit contractor, the MITRE Corporation, that it said owns the platform where the data resides.

The nonprofit does work for U.S. intelligence agencies and is the main contractor for the Conflict Observatory. The Yale lab has a contract under it. The MITRE Corporation said in a statement on Wednesday afternoon that the State Department formally terminated all work at the Conflict Observatory on Feb. 26.

“To the best of MITRE’s knowledge and belief,” it said, “the research data that was compiled has not been deleted and is currently maintained by a former partner on this contract.”

The Yale lab did research into abducted children and the “filtration sites” they and others were taken to in Russian-occupied Ukraine, where Ukrainians were interrogated and prepared for deportation to Russia. The researchers used open-source information and commercial satellite imagery.

Yale researchers were compiling the database, called Caesar, so the State Department could share information on abducted children with Europol and the International Criminal Court, which could eventually bring more war crimes charges against Russian officials. In 2022, after Russia began its full-scale invasion of Ukraine, President Joseph R. Biden Jr. accused the Russians of committing genocide. In March 2023, the court issued arrest warrants for President Vladimir V. Putin of Russia and the country’s children’s rights commissioner, Maria Lvova-Belova, for deporting children from Ukraine.

Ukrainian officials say Russia has abducted 20,000 children from Ukraine. Yale researchers have tracked more than 30,000 children to sites outside Ukraine. They have put information into the database on 6,000 children taken to Russia and more than 2,400 to Belarus. The database has detailed information on 314 kidnapped children in Russia’s system of coerced adoption, orphanages and foster care: their names and photographs, and dossiers of 20 to 30 pages on each child.

Some of the findings were previously disclosed in public reports from Yale. The center also gave information on the children to the Ukrainian government.

In July 2023, a Russian official said Russia had brought 700,000 children from conflict zones in Ukraine to Russia.

The Yale researchers have not been able to work on the project and gain access to the database since the funding freeze began in late January. When the U.S. government halted weapons aid and intelligence sharing with Ukraine after President Trump berated the country’s president, Volodymyr Zelensky, at the White House on Feb. 28, the Yale researchers lost access to satellite imagery.

The Trump administration restarted the intelligence sharing and weapons aid after a meeting in Saudi Arabia this month between U.S. and Ukrainian officials. But the Yale researchers still do not have access to satellite images.

Mr. Trump is trying to align with Mr. Putin, and the two spoke on the phone on Tuesday. Mr. Trump said he wanted to arrange a 30-day cease-fire in Ukraine, which the Ukrainians have agreed to, but Mr. Putin said he would only halt strikes temporarily on energy infrastructure.

Details of the State Department’s termination of its contracts for the research into potential Russian war crimes in Ukraine were reported earlier by The i Paper, a British news site, and The New Republic. The Washington Post first reported details of the congressional draft letter on Tuesday.