WASHINGTON — Michael Sussmann, a prominent cybersecurity lawyer with ties to Democrats, was acquitted on Tuesday of a felony charge that he lied to the F.B.I. about having no client in 2016 when he shared a tip about possible connections between Donald J. Trump and Russia.
The verdict was a blow to the special counsel, John H. Durham, who was appointed by the Trump administration three years ago to scour the Trump-Russia investigation for any wrongdoing.
The case centered on odd internet data that cybersecurity researchers discovered in 2016 after it became public that Russia had hacked Democrats and Mr. Trump had encouraged the country to target Hillary Clinton’s emails.
The researchers said the data might reflect a covert communications channel using servers for the Trump Organization and Alfa Bank, a Kremlin-linked bank. The F.B.I. briefly looked at the suspicions and dismissed them.
On Sept. 19, 2016, Mr. Sussmann brought those suspicions to a senior F.B.I. official. Prosecutors accused him of falsely telling the official that he was not there on behalf of any client, concealing that he was in fact working for both Mrs. Clinton’s campaign and a technology executive who had brought him the tip.
Mr. Durham and his trial team used court filings and trial testimony to detail how Mr. Sussmann, while working for a Democratic-linked law firm and logging his time to the Clinton campaign, had been trying to get reporters to write about the Alfa Bank suspicions.
But trying to persuade reporters to write about such suspicions is not a crime. Mr. Sussmann’s guilt or innocence turned on a narrow issue: whether he made a false statement to a senior F.B.I. official at the 2016 meeting, by saying he was sharing those suspicions on behalf of no one but himself.
Mr. Durham used the case to put forward a larger conspiracy: that there was a joint enterprise to essentially frame Mr. Trump for collusion with Russia by getting the F.B.I. to investigate the suspicions so reporters would write about it — a scheme involving the Clinton campaign; its opposition research firm, Fusion GPS; Mr. Sussmann; and a cybersecurity expert who brought the odd data and analysis to him.
That insinuation thrilled supporters of Mr. Trump who share his view that the Russia investigation was a “hoax,” and have sought to conflate the actual inquiry with sometimes thin or dubious allegations. In reality, the Alfa Bank matter was a sideshow: The F.B.I. had already opened its inquiry on other grounds before Mr. Sussmann passed on the tip, and the final report by the special counsel, Robert S. Mueller III, made no mention of the Alfa Bank suspicions.
But the case Mr. Durham and his team used to float their broad insinuations was thin — one count of making a false statement in a meeting with no other witnesses or contemporaneous notes. The evidence and arguments the lead prosecutor, Andrew DeFilippis, and his colleagues marshaled fell flat with the 12 jurors, who voted unanimously to find Mr. Sussmann not guilty.
Some supporters of Mr. Trump had been bracing for that outcome, pointing to the District of Columbia’s reputation as a heavily Democratic area and putting forward the prospect that a jury might be politically biased against a Trump-era prosecutor trying to convict a defendant who was working for the Clinton campaign.
Mr. Durham expressed disappointment about the verdict but said he respected the decision by the jury, which deliberated for about six hours.
“I also want to recognize and thank the investigators and the prosecution team for their dedicated efforts in seeking truth and justice in this case,” he said in a statement.
Shortly after the verdict, Mr. Sussmann read a brief statement to reporters outside the courthouse, expressing gratitude to the jury, his defense team and those who supported him and his family during what had been a difficult year. He did not take any questions.
“I told the truth to the F.B.I., and the jury clearly recognized that with their unanimous verdict today,” he said, adding: “Despite being falsely accused, I am relieved that justice ultimately prevailed in this case.”
The judge told the jury that they were not to take any of their own political views into account when deciding the facts.
The defense, which portrayed prosecutors’ insinuations as “political conspiracy theories,” had argued that Mr. Sussmann only brought the matter to the F.B.I. when he thought The New York Times was already on the cusp of writing an article about the matter, to give the bureau a heads-up so it would not be caught flat-footed.
Clinton campaign officials testified during the trial they had not told or authorized him to go to the F.B.I. — and that doing so was against their interests because they did not trust the bureau and it could slow down the publication of any article.
In a statement, Sean Berkowitz and Michael Bosworth, two of Mr. Sussmann’s defense lawyers, criticized Mr. Durham for bringing the indictment.
“Michael Sussmann should never have been charged in the first place,” they said. “This is a case of extraordinary prosecutorial overreach. And we believe that today’s verdict sends an unmistakable message to anyone who cares to listen: Politics is no substitute for evidence, and politics has no place in our system of justice.”
This story is developing. Check back for updates.