WASHINGTON — A lawyer for Senator Clinton’s 2016 presidential campaign was acquitted Tuesday of lying to the FBI when he pushed information meant to cast suspicions on President Trump and Russia in the run-up to the 2016 election.

The jury in the case of Michael Sussmann deliberated on Friday afternoon and Tuesday morning before reaching its verdict.

The case was the first courtroom test of special counsel John Durham since his appointment three years ago to search for government misconduct during the investigation into potential ties between Russia and Mr. Trump’s 2016 campaign. The verdict represents a setback for Mr. Durham’s work, especially since Mr. Trump’s supporters had looked to the probe to expose what they contend was sweeping wrongdoing by the FBI.

The trial focused on whether Mr. Sussmann, a cybersecurity attorney and former federal prosecutor, concealed from the FBI that he was representing Ms. Clinton’s campaign when he presented computer data that he said showed a possible secret backchannel between Russia-based Alfa Bank and Mr. Trump’s business company, the Trump Organization. The FBI investigated but quickly determined that there was no suspicious contact.

The bureau’s then-general counsel and the government’s star witness, James Baker, testified that he was “100% confident” that Mr. Sussmann had told him that he was not representing any client during the meeting. Prosecutors say he was actually acting on behalf of the Clinton campaign and another client, and that he hid that information so as to make it seem more credible and to boost the chances of getting the FBI to investigate.

Lawyers for Mr. Sussmann deny that he lied, saying that it was impossible to know with certainty what he told Mr. Baker since they were the only participants in the meeting and neither of them took notes.

They argued that if Mr. Sussmann said he wasn’t acting on the Clinton campaign’s behalf that that was technically accurate since he didn’t ask the FBI to take any particular action. And they said that even if he did make a false statement, it was ultimately irrelevant since the FBI was already investigating Russia and the Trump campaign and would have looked into the Alfa Bank data no matter the source.