“Please ask Assange for any State or HRC e-mail from August 10 to August 30--particularly on August 20, 2011,” Mr. Stone wrote to Randy Credico, a New York radio personality who had interviewed Mr. Assange several weeks earlier. Mr. Stone, a longtime confidant of Mr. Trump, had no formal role in his campaign at the time.

After earlier asserting his Fifth Amendment right against self-incrimination in the House probe, Mr. Credico now says he is willing to talk with investigators. He said he met on Wednesday with the committee’s Democratic staff members for what he called a limited conversation about WikiLeaks, the 2016 campaign and Mr. Stone.

As Mr. Credico has become more vocal about what he says are discrepancies in Mr. Stone’s account, Mr. Stone has responded with a series of threats, according to emails and text messages reviewed by the Journal.

In early April, in one of those emails, Mr. Stone accused Mr. Credico of serving as an informant.

“Everyone says u are wearing a wire for Mueller,” the April 7 email said. Two days later, Mr. Stone wrote: “Run your mouth = get sued.” Mr. Credico denies being an informant.

Mr. Stone said he was warning Mr. Credico against defaming him and urging him to “simply tell the truth.”