In March 2017, for instance, Mr. Stone acknowledged that before the 2016 election he traded private messages with Guccifer 2.0, the mysterious online avatar that was instrumental in helping WikiLeaks release internal emails and other political documents that eventually proved damaging to Hillary Clinton’s presidential bid and to the Democratic Party.

In an indictment unsealed last month, Mr. Mueller charged that Guccifer 2.0 was in fact a front for Russian intelligence officers. The indictment also said that a person “in regular contact with senior members” of Mr. Trump’s campaign had communicated with Guccifer 2.0. Government officials have identified that person as Mr. Stone.

Though Mr. Stone has downplayed his ties to Guccifer 2.0 as minimal and innocuous, media organizations have reported that he was also in direct communication with WikiLeaks in the weeks before the election, despite his assertions to the contrary. In February, The Atlantic published an article that included screenshots of private messages that Mr. Stone swapped with a Twitter account for WikiLeaks, a development that seemed at odds with a statement Mr. Stone gave to Congress in September when he claimed that he had communicated with the group only through an intermediary.

What further light can Ms. Davis shed on Mr. Stone’s activities?

It’s hard to say at this point, but Ms. Davis may be able to help investigators sort through Mr. Stone’s confusing and complicated ties to the two entities — WikiLeaks and Guccifer 2.0 — that are at the center of the hacking of Democratic emails and political documents, and their subsequent leak to the public.

Ms. Davis is close enough to Mr. Stone that, by his own account, he is the godfather of her son. And beyond the work he did for her during her run for governor, Mr. Stone employed her, on and off, for years, as an assistant in his office. Ms. Davis also has ties to one of Mr. Stone’s close aides, Andrew Miller, who served as her campaign manager when she ran for governor. Mr. Miller has himself been subpoenaed to appear before the grand jury in the Russia investigation.

Mr. Stone has nonetheless claimed Ms. Davis is unlikely to be helpful to Mr. Mueller. “She knows nothing about Russian collusion,” he said last week.

What does all of this mean for the larger investigation?

If Mr. Stone is ultimately charged with a crime, it would mean another person close to Mr. Trump had been felled by Mr. Mueller’s investigation. But beyond Mr. Stone’s actions, prosecutors are presumably also interested in what Mr. Trump knew about the hacks and leaks and when he knew it. Mr. Stone might be a key witness.