Political | Process

It is the first trial brought by special counsel John Durham, who was appointed by then-Attorney General William Barr to examine the early stages of the FBI’s investigation into the Trump campaign and whether it had any links to Russia’s efforts to interfere in the 2016 election.

Mr. Sussmann, who represented both Mr. Joffe and Mrs. Clinton’s presidential campaign, has pleaded not guilty and has said he brought the research to the FBI out of civic concern, as he told an agency official at the time.

Prosecutors allege he fed the information to the FBI on behalf of his clients. Mr. Joffe hasn’t been charged with any wrongdoing.

Federal prosecutors say they are investigating whether Mr. Joffe may have misused sensitive internet data in 2016 to draw a spurious computer connection between Mr. Trump’s company and a Russian bank—an allegation Mr. Joffe’s lawyer has denied.

Prosecutors have pressed witnesses and subpoenaed related documents on Mr. Joffe’s role in assembling that data, according to people familiar with the matter, and appear to be examining whether evidence shows Mr. Joffe knowingly manipulated or falsified any of it.

At a hearing last month, prosecutor Andrew DeFilippis of Mr. Durham’s team said the government probe was looking into whether Mr. Joffe’s work violated a law against defrauding the government in procurement.

“There is a Darpa contract—a federal contract—at issue here, which we have been looking at closely,” Mr. DeFilippis said, referring to the Defense Advanced Research Projects Agency, an arm of the Pentagon that funds research.

Darpa, through a spokesman, earlier this year denied any links to the research regarding the computer server connected to the Trump Organization. Mr. Joffe has denied any wrongdoing and his spokeswoman said the company where Mr. Joffe then worked, Neustar, Inc., never had a contract with Darpa. His lawyer has said in court filings that prosecutors are providing a misleading and incomplete narrative of Mr. Joffe’s actions.

“I have observed and experienced behavior by and from your team that one might expect from sharp-elbowed civil litigants, but not from experienced prosecutors who wield great power and are held to a more rigorous standard of proof,” a lawyer for Mr. Joffe wrote in an October letter to prosecutors reviewed by The Wall Street Journal.