WASHINGTON — The Trump-era special counsel scrutinizing the Russia investigation has acquired additional evidence that may bolster his case against a Democratic-linked lawyer accused of lying to the F.B.I. at a September 2016 meeting about Donald J. Trump’s possible ties to Russia, a new court filing revealed.
In the politically high-profile case, the lawyer, Michael Sussmann, is facing trial next month on a charge that he falsely told an F.B.I. official that he was not at the meeting on behalf of any client. There he relayed suspicions data scientists had about odd internet data they thought might indicate hidden Trump-Russia links.
The new filing by the special counsel, John H. Durham, says that the night before Mr. Sussmann’s meeting, he had texted the F.B.I. official stating that “I’m coming on my own — not on behalf of a client or company — want to help the bureau.”
The charge against Mr. Sussmann, which he denies, is narrow. But the case has attracted significant attention because Mr. Durham has used filings to put forward large amounts of information, insinuating there was a conspiracy involving the Hillary Clinton campaign to amplify suspicions of Trump-Russia collusion. Mr. Durham has not charged any such conspiracy, however.
The disclosure of the text to the F.B.I. official in question, James A. Baker, then the bureau’s general counsel, was part of a flurry of late-night filings on Monday by prosecutors and the defense centering on what evidence and arguments the judge should permit in the trial.
At the same time, the filings suggest that the special counsel may use the trial to continue to examine larger efforts linked to the Clinton campaign that raised suspicions about potential collusion between the Trump campaign and Russia — including the so-called Steele dossier.
The dossier is a notorious compendium of opposition research about purported Trump-Russia ties, since revealed to be thinly sourced and dubious. It was written by Christopher Steele, a subcontractor for Fusion G.P.S., a research firm that Mr. Sussmann’s former law firm, Perkins Coie, had hired to scrutinize such matters.
Mr. Sussmann, a cybersecurity specialist, had worked for the Democratic Party on issues related to Russia’s hacking of its servers. One of his partners at Perkins Coie, Marc Elias, a campaign law specialist, was representing the Clinton campaign and hired Fusion G.P.S.
Mr. Durham’s new filing refers to the dossier and Mr. Steele — including a meeting with Mr. Sussmann that Mr. Steele has said involved the suspicions about the odd internet data — and Mr. Sussmann’s legal team said that Mr. Durham appears to be planning to bring up the dossier at the trial even though the indictment does not mention it.
Mr. Sussmann’s defense lawyers accused Mr. Durham of promoting a “baseless narrative that the Clinton campaign conspired with others to trick the federal government into investigating ties between President Trump and Russia,” asking the judge to block prosecutors from making arguments and introducing evidence related to the Steele dossier.
“But there was no such conspiracy; the special counsel hasn’t charged such a crime; and the special counsel should not be permitted to turn Mr. Sussmann’s trial on a narrow false statement charge into a circus full of sideshows that will only fuel partisan fervor,” they wrote.
The Durham team’s filing also asked the judge to bar the defense from making arguments and presenting evidence “that depict the special counsel as politically motived or biased based on his appointment” by the Trump administration.
“The only purpose in advancing these arguments would be to stir the pot of political polarization, garner public attention and, most inappropriately, confuse jurors or encourage jury nullification,” it said. “Put bluntly, the defense wishes to make the special counsel out to be a political actor when, in fact, nothing could be further from the truth.”
In the spring of 2019, the special counsel investigating the Trump campaign and Russia, Robert S. Mueller III, detailed “numerous links between the Russian government and the Trump campaign” but did not charge any Trump associate with conspiring with Russia. As Mr. Trump continued to claim that he was the victim of a “deep state” conspiracy, the attorney general at the time, William P. Barr, assigned Mr. Durham to scour the Russia investigation for any wrongdoing.
But Mr. Durham has not developed any cases against high-level officials. Instead, he has brought false-statements charges involving two efforts by outsiders to hunt for signs of Trump-Russia links, both of which were thin and involved Perkins Coie in some way. He has used the indictments to insinuate that the Clinton campaign may have orchestrated the concoction of false smears against Mr. Trump, but without charging such a conspiracy.
One such effort was the Steele dossier, and the other was the suspicions that Mr. Sussmann relayed to Mr. Baker. The latter suspicions had been developed by a group of data scientists who analyzed odd internet data they thought might suggest clandestine communications between a server for the Trump Organization and a server for Alfa Bank, a Kremlin-linked Russian financial institution.
The F.B.I. — which had already opened the investigation that would evolve into the Mueller inquiry — looked into the Alfa Bank matter but decided the suspicions were unfounded.
After Mr. Sussmann’s indictment, several criminal law specialists said the charge was an unusually thin basis for a federal case because it boiled down to a dispute over what was said at a one-on-one meeting at which there were no other witnesses and there was no recording. But the newly disclosed text message from Mr. Sussmann could bolster prosecutors’ case.
In accusing Mr. Sussmann of falsely saying he was not conveying the suspicions on behalf of any client, the indictment also contended that he was concealing that he was actually representing two clients at that meeting — the Clinton campaign and a technology executive, Rodney Joffe, who worked with the cyberspecialists who analyzed the Alfa Bank data. Law firm billing records show that Mr. Sussmann listed the campaign for time working on Alfa Bank issues.
Mr. Sussmann’s legal team has denied that he told Mr. Baker he was not conveying the information on behalf of any client. They also insisted to the Justice Department before the indictment that Mr. Sussmann was not there at the direction or on behalf of the campaign. In court filings, they have acknowledged that Mr. Sussmann “arranged for this meeting on behalf of his client,” referring to Mr. Joffe.
The defense for Mr. Sussmann therefore may turn in part on what it means to be somewhere on behalf of a client. In a separate filing on Monday night, the defense asked the judge, Christopher Cooper of the Federal District Court for the District of Columbia, to dismiss the case if Mr. Durham does not grant immunity to Mr. Joffe, so that the technology executive can testify about his interactions with Mr. Sussmann regarding the meeting.
In that filing, they said Mr. Joffe would offer “critical exculpatory testimony on behalf of Mr. Sussmann,” including that the two agreed that he should take the information to the F.B.I. “to help the government, not to benefit Mr. Joffe.” They also said that “contrary to the special counsel’s entire theory,” Mr. Joffe’s work with the data scientists was not connected to the campaign.
A spokeswoman for Mr. Joffe did not provide a comment. But a letter from Mr. Joffe’s lawyer included in the filings said that while Mr. Joffe “can provide exculpatory information concerning the allegations against” Mr. Sussmann, Mr. Joffe still faced the possible risk of indictment and would invoke his Fifth Amendment rights not to testify.