The defendant, Michael Sussmann, is accused of lying to the F.B.I. in a meeting about Trump and Russia. He denies wrongdoing.

WASHINGTON — The special counsel appointed by the Trump administration to scrutinize the Russia investigation obtained a grand jury indictment on Thursday of a prominent cybersecurity lawyer, accusing him of lying to the F.B.I. five years ago during a meeting about Donald J. Trump and Russia.

The indictment secured by the special counsel, John H. Durham, also made public his findings about an episode in which cybersecurity researchers identified unusual internet data in 2016 that they said suggested the possibility of a covert communications channel between the Trump Organization and Alfa Bank, a Kremlin-linked financial institution.

He concluded that the Clinton campaign covertly helped push those suspicions to the F.B.I. and reporters, the indictment shows. The F.B.I. looked into the questions about Alfa Bank but dismissed them as unfounded, and the special counsel who later took over the Russia investigation, Robert S. Mueller III, ignored the matter in his final report.

The charging of the lawyer, Michael A. Sussmann, had been expected. He is accused of falsely telling a top F.B.I. lawyer that he was not representing any client at the meeting about those suspicions. Prosecutors contend that he was instead representing both a technology executive and the Hillary Clinton campaign.

“Sussmann’s false statement misled the F.B.I. general counsel and other F.B.I. personnel concerning the political nature of his work and deprived the F.B.I. of information that might have permitted it more fully to assess and uncover the origins of the relevant data and technical analysis, including the identities and motivations of Sussmann’s clients,” the indictment said.

Mr. Sussmann’s defense lawyers, Sean Berkowitz and Michael Bosworth, have denied the accusation, insisting that he did not say he had no client and maintaining that the evidence against him is weak. They also denied that the question of who Mr. Sussmann was working for was material, saying the FB.I. would have investigated the matter regardless.

“Michael Sussmann was indicted today because of politics, not facts,” they said on Thursday. “The special counsel appears to be using this indictment to advance a conspiracy theory he has chosen not to actually charge. This case represents the opposite of everything the Department of Justice is supposed to stand for. Mr. Sussmann will fight this baseless and politically inspired prosecution.”

A former computer crimes prosecutor who worked for the Justice Department for 12 years, Mr. Sussmann in 2016 represented the Democratic National Committee on issues related to Russia’s hacking of its servers.

He has been a cybersecurity lawyer for 16 years at the law firm Perkins Coie, which has deep ties to the Democratic Party. A colleague of Mr. Sussmann’s, Marc Elias, was the general counsel to the Clinton campaign. He left the law firm last month.

The firm said in a statement on Thursday that Mr. Sussmann had also departed: “In light of the special counsel’s action today, Michael Sussmann, who has been on leave from the firm, offered his resignation from the firm in order to focus on his legal defense, and the firm accepted it.”

The charge against him centers on a Sept. 19, 2016, meeting with the F.B.I. lawyer, James A. Baker, in which Mr. Sussmann relayed concerns about the odd internet data. Cybersecurity researchers had said it might be evidence of clandestine communications channel between computer servers associated with the Trump Organization and with Russia’s Alfa Bank.

The case against Mr. Sussmann turns on Mr. Baker’s recollection that Mr. Sussmann told him he was not at the meeting on behalf of any client — which Mr. Sussmann denies saying. There were no witnesses to their conversation.

The indictment says Mr. Baker later briefed another F.B.I. official — apparently Bill Priestap, the bureau’s top counterintelligence official — about the meeting, and that Mr. Priestap’s notes say Mr. Baker recounted that Mr. Sussmann said he was “not doing this for any client.” (It is not clear whether such notes would be admissible at a trial.)

In 2017, Mr. Sussmann testified under oath to Congress that he was representing the unnamed technology executive, and his legal team agrees that executive was his client at the meeting — but the only one.

Internal law firm billing records, however, show that Mr. Sussmann had been logging his time on Alfa Bank matters to the Clinton campaign, the indictment says, contending that the campaign was his client, too. Those records are said to also show that Mr. Sussmann met or spoke with Mr. Elias about Alfa Bank repeatedly.

Seeking to head off any indictment, Mr. Sussmann’s defense lawyers had argued to the Justice Department that the billing records were misleading and that he was not at the meeting at the direction or on behalf of the Clinton campaign, according to people familiar with the case. They also denied that the records could be fairly interpreted as showing that he billed the meeting with Mr. Baker to the campaign, as the indictment accuses him.

Mr. Durham is known to have been closely scrutinizing the Alfa Bank episode since last fall, including using a grand jury to subpoena documents and question witnesses in ways that suggested he was pursuing a theory that the data had been manipulated or the analysis of it knowingly torqued.

The 27-page indictment disclosed much of what he found, including quoting extensively from internal communications of unnamed researchers.

The unidentified technology executive whom Mr. Sussmann represented was not the first researcher to scrutinize the data. But his company had access to large amounts of internet data, and he came to play an important role in driving the research and analysis, which he told Mr. Sussmann about around July 2016, the indictment said.

In August of that year, the technology executive outlined to other researchers the goal of the effort, saying that unspecified “VIPs” wanted to find “true” information that would merit closer scrutiny. Noting that Mr. Trump had claimed he had no interactions with Russian financial institutions, the executive wrote that data suggesting that was false “would be jackpot” and would “give the base of a very useful narrative.”

The executive also wrote: “Being able to provide evidence of *anything* that shows an attempt to behave badly in relation to this, the VIPs would be happy. They’re looking for a true story that could be used as the basis for closer examination.”

But one of the researchers working on the project worried that their analysis had weaknesses and that suggested they all shared anti-Trump sentiment.

“The only thing that drive[s] us at this point is that we just do not like” Trump, the indictment quoted one unnamed researcher as writing. “This will not fly in eyes of public scrutiny. Folks, I am afraid we have tunnel vision. Time to regroup?”

In early September, the indictment said, Mr. Sussmann met with a New York Times reporter who would later draft a story about Alfa Bank, and also began work on a so-called white paper that would summarize and explain the researchers’ data and analysis, billing the time to the Clinton campaign.

On Sept. 12, the indictment said, Mr. Sussmann called Mr. Elias, the Clinton campaign lawyer, and spoke about his “efforts to communicate” with the Times reporter about the Alfa Bank allegations. Both billed the call to the campaign. And three days later, Mr. Elias exchanged emails with top campaign officials about the matter.

In the meantime, on Sept. 14, five days before Mr. Sussmann met with the F.B.I., the technology executive emailed three researchers helping him with data. The executive sought to ensure the analysis they were assembling would strike security experts as simply “plausible,” even if it fell short of demonstrably true, prosecutors said.

Mr. Sussmann also continued to push the Alfa Bank story to reporters. A month before the election, as Times editors were weighing whether to publish an article the reporter had drafted, Mr. Sussmann told him he should show the editors an opinion essay saying the paper’s investigative reporters had not published as many stories regarding Mr. Trump as other media outlets, the indictment said.

Attorney General William P. Barr appointed Mr. Durham in May 2019 to scour the Russia investigation for any wrongdoing. Both Mr. Trump and Mr. Barr stoked expectations among Mr. Trump’s supporters that the prosecutors would uncover grave offenses by high-level government officials and support claims that the Russia investigation was a plot concocted by the so-called deep state to sabotage Mr. Trump.

To date, Mr. Durham’s investigation has fallen short of those expectations. Out of office, Mr. Trump has repeatedly issued statements fuming, “Where’s Durham?”

The current attorney general, Merrick B. Garland, said at his confirmation hearing in February that he would let Mr. Durham continue to work and told Congress in July that he agreed with Mr. Barr’s earlier direction that Mr. Durham should eventually submit a report in a form that could be made public.

Funding for most Justice Department operations, like much of the federal government, is controlled by an annual budget that covers a fiscal year that ends on Sept. 30. Spokesmen for Mr. Garland and Mr. Durham have declined to answer questions about whether Mr. Durham’s office has funding approval to continue operating beyond this month.

But in announcing the indictment of Mr. Sussmann, the Justice Department said, “The special counsel’s investigation is ongoing.”