An ex-FBI official testified on Thursday that a former lawyer for Hillary Clinton's presidential election campaign said he was not acting on behalf of any client when in 2016 he relayed a tip about Donald Trump's possible ties to Russia.

The Clinton campaign lawyer, Michael Sussmann, is accused of lying to the FBI by hiding his political ties, which he denies. Former FBI general counsel James Baker testified in federal court for several hours on Thursday about a September 2016 meeting with Sussmann.

At that meeting, Sussmann provided a since-discredited tip alleging internet communications between the business of Trump, who was subsequently elected president, and a Russian bank.

Baker said he is "100% confident" that Sussmann told him he was not attending the meeting on behalf of any client.

"I think it was pretty close to the beginning of the meeting. Part of his introduction to the meeting," Baker said.

Baker's testimony bolsters U.S. Special Counsel John Durham, the special prosecutor appointed during Trump's administration to probe any missteps in the FBI's investigation into whether Trump's campaign was colluding with Russia. The case against Sussmann is the first one brought by Durham to culminate in a jury trial.

Durham's team says Sussmann was representing two clients: Clinton's campaign and Rodney Joffe, a technology executive who oversaw the research into the alleged connections between Alfa Bank and the Trump Organization.

Sussmann has pleaded not guilty, and his lawyers have maintained his innocence, saying he never lied to Baker and did not arrange the meeting on behalf of his clients.

"No one told him to go. No one authorized him to go," said attorney Michael Bosworth during opening remarks to jurors on Tuesday.

President Joe Biden's administration has allowed Durham to continue his work.