The case against Mr. Sussmann centers on the question of who his client was when he conveyed certain suspicions about Mr. Trump and Russia to the F.B.I. in September 2016. Among other things, investigators have examined whether Mr. Sussmann was secretly working for the Clinton campaign — which he denies.
An indictment is not a certainty: On rare occasions, grand juries decline prosecutors’ requests. But Mr. Sussmann’s lawyers, Sean M. Berkowitz and Michael S. Bosworth of Latham & Watkins, acknowledged on Wednesday that they expected him to be indicted, while denying he made any false statement.
“Mr. Sussmann has committed no crime,” they said. “Any prosecution here would be baseless, unprecedented and an unwarranted deviation from the apolitical and principled way in which the Department of Justice is supposed to do its work. We are confident that if Mr. Sussmann is charged, he will prevail at trial and vindicate his good name.”
A spokesman for Attorney General Merrick B. Garland, who has the authority to overrule Mr. Durham but is said to have declined to, did not comment. Nor did a spokesman for Mr. Durham.
The accusation against Mr. Sussmann focuses on a meeting he had on Sept. 19, 2016, with James A. Baker, who was the F.B.I.’s top lawyer at the time, according to the people familiar with the matter. They spoke on condition of anonymity.
Because of a five-year statute of limitations for such cases, Mr. Durham has a deadline of this weekend to bring a charge over activity from that date.
At the meeting, Mr. Sussmann relayed data and analysis from cybersecurity researchers who thought that odd internet data might be evidence of a covert communications channel between computer servers associated with the Trump Organization and with Alfa Bank, a Kremlin-linked Russian financial institution.
The F.B.I. eventually decided those concerns had no merit. The special counsel who later took over the Russia investigation, Robert S. Mueller III, ignored the matter in his final report.