The State Department has ended funding for the tracking 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 that U.S. lawmakers plan to send to Secretary of State 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, Secretary of State Marco 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 congressional letter, organized by Representative Greg Landsman, Democrat of Ohio, said “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.

“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 work of the Yale Center 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.

The 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 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.”

Ukrainian officials say Russia has abducted 20,000 children from Ukraine. Yale researchers said in earlier reports they have tracked 30,000 children to sites outside of 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: 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.

The main contractor for the State Department project is the MITRE Corporation, a nonprofit that mainly does work for the U.S. government, including for intelligence agencies. The Yale lab had a contract under it.

The State Department did not reply to a request for comment about the project and the status of the database. The MITRE Corporation also did not reply.

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 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 Volodymyr Zelensky at the White House on Feb. 28, the president of Ukraine, 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 President Vladimir V. Putin of Russia, and the two spoke on the phone on Tuesday. Mr. Trump said he wants 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 earlier reported 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.