WASHINGTON — An analyst who was a key contributor to Democratic-funded opposition research into possible links between Donald J. Trump and Russia was arrested on Thursday and charged with lying to the F.B.I. about his sources.

The analyst, Igor Danchenko, was a primary researcher for claims that went into the so-called Steele dossier, a compendium of rumors and unproven assertions suggesting that Mr. Trump and his 2016 campaign were compromised by and conspiring with Russian intelligence officials to help him defeat Hillary Clinton.

In a 39-page indictment obtained by the special counsel, John H. Durham, a grand jury accused Mr. Danchenko of five counts of making false statements to the F.B.I. about his sources for certain claims in the dossier.

The indictment showed that two and a half years after then-Attorney General William P. Barr appointed him to scour the Trump-Russia investigation for any wrongdoing, and a year after Mr. Trump lost re-election, Mr. Durham continues to press ahead.

“The special counsel’s investigation is ongoing,” the Justice Department said in a release that described the indictment but provided no direct statement from Mr. Durham.

Mr. Danchenko appeared before a magistrate judge at the federal courthouse in Alexandria, Va., on Thursday afternoon, wearing a white shirt and dark green pants and standing with his hands behind his back as he listened to the proceeding. His defense lawyer tried to enter a plea of not guilty, but the judge said that was premature before releasing Mr. Danchenko on bond. The lawyer declined to make a statement to reporters afterward.

The dossier has played a vivid role in the Trump-Russia affair, but was largely peripheral to the official inquiry. The F.B.I. had already opened its counterintelligence investigation into the Trump campaign and Russia before the Steele dossier reached the agents working on that matter. The special counsel who eventually took over the inquiry into Russian interference in the 2016 election, Robert S. Mueller III, did not rely upon it in his final report.

But some claims from the dossier made their way into an F.B.I. wiretap application targeting a former Trump campaign adviser, Carter Page, in October 2016, and three renewal applications the following year. And other portions of it — particularly a salacious claim about a purported blackmail tape — caused a political and media firestorm when Buzzfeed published the materials in January 2017, shortly before Mr. Trump was sworn in.

Most of the important claims in the dossier — a series of reports written by Mr. Danchenko’s employer, Christopher Steele, a former British intelligence agent — have not been proven, and some have been refuted, including by Mr. Mueller. F.B.I. agents interviewed Mr. Danchenko several times in 2017 when they were seeking to run down the claims.

The first false statement charge in the indictment concerns Mr. Danchenko's interactions with what the indictment describes as a public relations executive with strong ties to the Democratic Party.

The indictment said Mr. Danchenko falsely told the F.B.I. that he had not discussed the claims in the dossier with the public relations executive. But, the indictment said, the executive — who in his professional career frequently interacted with Eurasian clients, with a particular focus on Russia — was a source for some of the claims, including gossip about the ouster of Paul Manafort as Mr. Trump’s campaign chairman.

The indictment did not name the executive, whom it linked to the dossier in several other ways. It said the executive had lunch with Mr. Danchenko in Moscow in June 2016. At the time, the executive was staying in the same Moscow hotel where the dossier claimed that Russian intelligence made a blackmail sex tape involving Mr. Trump and prostitutes.

The executive toured the presidential suite, the indictment said, and a hotel staff member told him that Mr. Trump had stayed there — but the executive and another person on the tour told the F.B.I. that the staff member did not mention any salacious activity.

Given that the executive was present at places and events where Mr. Danchenko collected information for the dossier, the indictment said, the researcher’s “subsequent lie” about that executive’s connection to it “was highly material to the F.B.I.’s investigation of these matters.”

The other four false-statement charges concern Mr. Danchenko’s claims to the F.B.I. about purported interactions with Sergei Millian, a former president of the Russian-American Chamber of Commerce, as a potential source for the dossier. (The indictment did not explicitly name Mr. Millian, who has previously said that Mr. Danchenko reached out to him but that he never responded or spoke with the researcher.)

Mr. Danchenko told the F.B.I. that he received a phone call in late July 2016 from a Russian-sounding person who did not identify himself but whom Mr. Danchenko took to be Mr. Millian, and that he had arranged to meet the businessman in New York but Mr. Millian did not show up.

But the indictment said it was not true that Mr. Danchenko believed he had spoken to Mr. Millian in late July. It cited an email Mr. Danchenko sent to Mr. Millian in August, following up on an earlier email to which Mr. Millian had apparently not responded. The wording of that August email was inconsistent with the claim of a recent phone call, the indictment said.

The flaws in the Steele dossier and Mr. Danchenko’s 2017 interviews with F.B.I. agents played a central role in a high-profile 2019 report by the Justice Department’s inspector general.

That report cited ways in which Mr. Danchenko’s description of his sourcing suggested the material was thinner and more speculative than how Mr. Steele later drafted them, and it faulted the F.B.I. for continuing to cite material from the dossier in wiretap renewal applications without alerting judges that a reason had arisen to doubt its credibility.

The inspector general report also said that a decade earlier, when Mr. Danchenko — who was born in Russia but lives in the United States — worked for the Brookings Institution, a prominent Washington think-tank, he had been the subject of a counterintelligence investigation into whether he was a Russian agent.

In an interview with The New York Times in 2020, Mr. Danchenko defended the integrity of his work, saying he had been tasked to gather “raw intelligence” and was simply passing it on to Mr. Steele. Mr. Danchenko — who made his name as a Russia analyst by exposing indications that the dissertation of President Vladimir V. Putin of Russia contained plagiarized material — also denied being a Russian agent.

“I’ve never been a Russian agent,” Mr. Danchenko said. “It is ridiculous to suggest that. This, I think, it’s slander.”

The indictment did not accuse Mr. Danchenko of working for Russian intelligence, although Mr. Durham did quote a June 2016 email from the public relations executive to an acquaintance in which the executive said he thought Mr. Danchenko had worked for a Russian intelligence agency “since he told me he spent two years in Iran. And when I first met him, he knew more about me than I did.” The line was followed by a winking emoticon.

Mr. Steele’s efforts were part of opposition research that Democrats were indirectly funding by the time the 2016 general election took shape.

His business intelligence firm was a subcontractor to another research firm, Fusion GPS, which originally began researching Trump-Russia ties during the primary election on behalf of a conservative who opposed Mr. Trump’s campaign. When it was clear that Mr. Trump would be the Republican nominee, the original funder dropped the effort, but Fusion GPS was hired to keep going by the Perkins Coie law firm, which was working for the Hillary Clinton campaign.

Mr. Danchenko has said he did not know who Mr. Steele’s client was at the time and considered himself a nonpartisan analyst and researcher.

In February, Mr. Durham used a subpoena to obtain old personnel files and other documents related to Mr. Danchenko from the Brookings Institution, where Mr. Danchenko had worked from 2005 until 2010.

The charges against Mr. Danchenko follow Mr. Durham’s indictment in September of a cybersecurity lawyer, Michael A. Sussmann, which accused him of lying to the F.B.I. about who he was working for when he brought concerns about possible Trump-Russia links to the bureau in September 2016.

Mr. Sussmann, who then also worked for Perkins Coie, was relaying concerns from data scientists about odd internet logs that they said suggested the possibility of a covert communications channel between the Trump Organization and Alfa Bank, a Kremlin-linked financial institution. He has denied lying to the F.B.I. about who he was working for.

William K. Rashbaum and Christopher Cameron contributed reporting.