The F.B.I. cut ties to at least a handful of informants and issued warnings about dozens of others after an internal review prompted by concerns that they were linked to Russian disinformation, current and former U.S. officials said.

The review was carried out in 2020 and 2021 by a small group within the bureau’s counterintelligence division, with the findings then passed along to field offices, which handle informants.

It led to the severing of sources — some of whom had offered information about Russia-aligned oligarchs, political leaders and other influential figures — at a moment when the bureau was asking agents to produce more information from and about those same networks. The review was conducted during and after the 2020 election, when concerns about Russian meddling were running high, and at a time when the United States was closely monitoring whether Russia would invade Ukraine.

The episode highlighted a tricky balance: The more access informants have to valuable intelligence, the higher the risk that they could knowingly or unknowingly be used to channel disinformation. This is particularly true with regard to post-Soviet countries, where shifting alliances among oligarchs, politicians and intelligence services have far-reaching consequences that can be difficult for Western governments to discern.

Even in an age of high-tech intelligence gathering and surveillance, human sources continue to play an important role in law enforcement and national security, giving agents the chance to gather insights and perspective that cannot always be gleaned from communications intercepts, for example.

The New York Times has independently confirmed, but is not disclosing, the identities of several of the F.B.I. informants who provided information about Russia and Ukraine and who were cut off around the time of the review by the bureau’s counterintelligence division, including one informant that predated the review.

Johnathan C. Buma, an F.B.I. agent who oversaw at least four of the informants who were dropped, suggested in a written statement provided to the Senate Judiciary Committee last year that law enforcement should embrace the murkiness that comes with operating in the shadows.

“Typical disinformation operations are based on partial truths, and the only way to determine the veracity of the allegations is to conduct an independent investigation to attempt corroboration,” Mr. Buma wrote in explaining his opposition to the terminations.

His statement to the Senate Judiciary Committee, which is led by Democrats, as well as a statement Mr. Buma submitted earlier to a special subcommittee of the Republican-controlled House, came after he filed a whistle-blower complaint accusing the F.B.I. of suppressing intelligence from his sources and retaliating against him.

The F.B.I. is investigating Mr. Buma’s dealings with an informant he worked with after the bureau cut off those identified in the counterintelligence review, a person familiar with the matter said.

The F.B.I. had been aware of Russian disinformation efforts for years, and eventually became concerned that the campaign extended to its own informants.

In particular, the F.B.I. watched as informants across the bureau’s different divisions began peddling new information that was politically explosive. It included reports regarding President Biden’s family and former President Donald J. Trump, as well as other inflammatory topics, according to former and current U.S. officials and an ex-informant for the counterintelligence division.

The types of concerns that prompted the review spilled into public view in February, when prosecutors indicted a longtime informant on Russia and Ukraine matters, Alexander Smirnov, for lying to the F.B.I.

Prosecutors accused him of fabricating claims about bribes paid to the Bidens by a Ukrainian energy company whose board included the president’s son, Hunter Biden. Prosecutors said Mr. Smirnov had passed along information about Hunter Biden — though they did not provide specifics — from Russian intelligence.

Mr. Smirnov was flagged as part of the F.B.I. review but he was not shut down, because information he was providing was being used in other investigations, the former and current U.S. officials said.

Around the time of the review, the F.B.I. circulated internal memos to agents hinting at competing imperatives. On the one hand, agents were instructed to gather more intelligence from informants about Russian efforts to meddle in U.S. politics, and to retaliate against the United States for its support of Ukraine.

On the other, they were urged to be on the lookout for disinformation, misinformation or influence operations from foreign governments that took aim at American politics, according to the memos, which were obtained by The Times.

The memos, each of which was labeled “collection priorities message,” listed the identification numbers and handling agents of informants who could be of assistance on such matters. The memos do not mention the terminations, or any concerns about specific informants.

A former official said that dozens of F.B.I. agents in field offices were warned to handle their informants, known as confidential human sources, with extra care because the Russians might have been aware of their contact with the United States. Under bureau policy, the decision to end relationships with informants rests with the F.B.I. field offices and not headquarters.

A U.S. official described this effort as an “awareness campaign” inside the F.B.I.

The bureau’s sources are often encouraged to maintain associations with criminal figures or foreign intelligence services. The idea is for them to report back on those associates; in the process, though, they can become conduits used by those associates to inject false information — intentionally or unknowingly — into the realms of U.S. law enforcement or intelligence.

Some terminations in early 2022 were classified as precautionary and not for cause, according to Mr. Buma’s statement and one of his former informants. That suggests there was no specific evidence that those informants had willfully tried to channel Russian disinformation into federal law enforcement, but rather that there was concern that they might have done so unwittingly, or merely been associated with people believed to be pushing disinformation, or politically motivated information.

Information provided by one of Mr. Buma’s terminated informants, an American businessman with deep connections overseas, was used by the special counsel investigating Russian meddling in the 2016 election, according to Mr. Buma’s statement to the Senate Judiciary Committee.

Other information from the businessman was used to revoke the U.S. visa of a Ukrainian-Russian oligarch and to support the decision to impose sanctions on a Ukrainian oligarch who had been a key backer of President Volodymyr Zelensky of Ukraine, according to Mr. Buma’s statement. And it was used to identify two corrupt federal law enforcement agents.

Among the associations that appear to have raised red flags within the F.B.I. was the businessman’s recruitment of two Ukrainians who would themselves become F.B.I. informants. One of the Ukrainians was a former K.G.B. agent who had become a Ukrainian intelligence operative, who developed high-level Ukrainian government contacts through his leadership of a foundation dedicated to tracking kleptocracy, according to Mr. Buma’s statement. It identified the other as a researcher for the foundation who had a background in economics.

In January 2019, according to interviews and Mr. Buma’s statement, the two Ukrainians traveled to the Los Angeles area for meetings during which they provided information to representatives from the F.B.I. and other agencies about oligarchs, money laundering and Ukrainian and American political figures.

Among their claims was one that Hunter Biden had failed to disclose lobbying he did for the Ukrainian energy company Burisma, and had failed to pay taxes on income from the company. Mr. Biden was not charged with lobbying violations. He was charged last year with failure to file tax returns covering millions of dollars in income from Burisma and other foreign businesses. It is not clear whether information from the two Ukrainian informants played any role in the investigation.

The F.B.I. first pressed to cut off the businessman after he and the two Ukrainians attended a conservative gala in May 2019. At the event, the Ukrainians presented a thumb drive containing allegations about Mr. Biden and other Democrats to an aide traveling with Mike Pompeo, then the secretary of state, according to internal F.B.I. reports and an article published in Business Insider.

Mr. Buma successfully resisted efforts to terminate the American businessman.

Mr. Buma argued that the informant was granting the F.B.I. a critical view into a murky world that was increasingly important to U.S. national security as Russia built up its efforts to influence American politics and exert control over Ukraine, according to interviews and the statement Mr. Buma provided to the Senate Judiciary Committee.

Mr. Buma had been trained by the bureau to speak Russian. Part of his job was identifying and recruiting informants with access to Russian and Ukrainian oligarchs, politicians and their networks.

The American businessman became “one of the F.B.I.’s top C.H.S.s whose reporting had been extensively corroborated through predicated investigations, with numerous well-documented high-impact successes related to countering foreign influence and public corruption on both sides of the political spectrum,” Mr. Buma wrote in his statement to the Senate, referring to confidential human sources.

Yet, in the weeks before Russia’s invasion of Ukraine, the bureau again expressed concerns about the businessman and other sources connected to him.

In a meeting in February 2022, an official with the bureau’s Foreign Influence Task Force told Mr. Buma that he was “not the only field agent whom they were asking to close their sources related to Russia/Ukraine matters just as the war erupted,” Mr. Buma wrote in his statement to the Senate. “When I questioned the wisdom of their request, the supervising analyst claimed their recommendation relied on highly classified information from the National Security Agency.”

The informants were closed out, as were others linked to the businessman, including, Mr. Buma recalled in his statement, “many other productive sources in that category who took years for me to develop.”

Mr. Buma suggested in his statement that the closures were an effort to shut down investigations that might implicate Trump allies, including Rudolph W. Giuliani. Mr. Buma had collected information from the businessman about Mr. Giuliani’s efforts to damage the Bidens by highlighting their work in Ukraine.

The F.B.I. declined to comment on Mr. Buma’s claims.

Mr. Buma privately discussed his allegations last summer with Republican staff members for the House subcommittee and with aides to Senator Sheldon Whitehouse, a Rhode Island Democrat who chairs the oversight subcommittee of the Senate Judiciary Committee.

There is no evidence that either congressional committee is investigating his claims. A spokesman for the House subcommittee declined to comment, while representatives for the Senate Judiciary Committee and Mr. Whitehouse did not respond.

Months later, Mr. Buma’s home was searched for classified information by the F.B.I. Mr. Buma has been suspended from the bureau, but he has not been criminally charged.

Scott Horton, a lawyer for Mr. Buma, cast the investigation as “revenge” against his client for having suggested that the F.B.I.’s handling of confidential sources was affected by political bias against the Bidens and in favor of Mr. Trump’s allies.

Mr. Horton said he had met with Hunter Biden’s lawyers to discuss how Mr. Buma’s story might be of assistance. Another lawyer for Mr. Buma, Mark Geragos, is also representing Mr. Biden.

The phenomenon of “misnaming,” or calling someone by the wrong name, is more common than we think. Pexels, Public Domain

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