Roger Stone pauses while speaking at the American Priority Conference on Dec. 6 in Washington. The longtime Trump adviser has been under scrutiny for months by special counsel Robert S. Mueller III. (Drew Angerer/Getty Images)

The House Intelligence Committee voted unanimously Thursday to turn over a transcript of Roger Stone’s testimony to special counsel Robert S. Mueller III — a key step that would assist prosecutors if they pursue charges in their investigation of the longtime Trump adviser.

The vote took place behind closed doors but was confirmed by a person with knowledge of the action.

Last Friday, Mueller requested an official transcript of Stone’s 2017 testimony — the first time the special counsel had asked the committee to turn over material it has gathered during its investigation of Russian interference in the 2016 presidential campaign.

Mueller has had access to an unofficial copy of Stone’s testimony for weeks, but legal experts said he would need a certified copy provided by the committee if he wished to use the document to seek an indictment.

Stone, who has advised Donald Trump on and off for decades and was in contact with him during the 2016 campaign, has been a focus of the special counsel as Mueller investigates whether the Trump campaign had advance knowledge of WikiLeaks’s release of Democratic emails allegedly hacked by Russian operatives.

Stone and WikiLeaks have repeatedly denied any coordination. On Thursday, Stone’s attorney Grant Smith reiterated that denial in a letter to House Intelligence Committee Chairman Devin Nunes (R-Calif.), writing, “Mr. Stone never had advance knowledge of the source or content of any releases by WikiLeaks or other organizations, and no person can prove, or truthfully claim, otherwise.”

Mueller’s request for the transcript of Stone’s testimony suggests that he may be moving quickly to wrap up his investigation of the longtime Trump confidant. Mueller could have waited until next month to request the transcript, when control of the House will go to Democrats, who have said they wish to provide the special counsel with transcripts of interviews of dozens of witnesses.

The committee’s decision Thursday to accede with his request indicates that although Republicans have followed Trump’s lead in attacking the Mueller inquiry, they are not willing to take formal action to block it.

It is unclear what aspect of Stone’s testimony Mueller is scrutinizing. But Stone has given conflicting accounts about what prompted him to accurately predict during the 2016 race that WikiLeaks was going to unleash material that would hurt Democratic presidential nominee Hillary Clinton.

“I believe that there’s ample reason to be concerned about his truthfulness,” Rep. Adam B. Schiff (Calif.), the ranking Democrat on the committee, said Sunday on NBC’s “Meet the Press.” “And I do think that with respect to Mr. Stone, and perhaps others, the special counsel is in a better position to determine the truth or falsity of that testimony, and that we ought to provide it to the special counsel.”

Mueller could use the threat of a false-statement charge to seek cooperation from Stone, as Mueller has done with other Trump advisers, such as former national security adviser Michael Flynn and longtime Trump lawyer Michael Cohen.

In an interview Wednesday, Stone said he had not been notified of Mueller’s request. But he said he is confident that the transcript of his testimony will not provide the special counsel with grounds to charge him.

“I don’t think any reasonable attorney who looks at it would conclude that I committed perjury, which requires intent and materiality,” Stone said.

Smith, Stone’s lawyer, sent a long letter to Nunes on Thursday demanding that the transcript of Stone’s testimony be released publicly and accusing Schiff of “exploiting” committee secrecy rules to “misrepresent” what he said in the closed-door session.

“What has become evident since the investigation began is that the Democrats’ utter failure to substantiate their pre-determined narrative concerning ‘Russian collusion’ and their continuing inability to prove the allegations and claims that they had contrived from the outset, on the basis of nothing more than purely partisan-motivated prejudgments, has led them to desperate measure[s] which have degenerated this investigation into a patently-malicious effort to smear people, ruin lives, and ignore glaring evidence of related misconduct by a number of public officials clearly warranting investigation by your Committee, at minimum,” Smith wrote.

A spokesman for Schiff declined to comment.

Karoun Demirjian, Tom Hamburger and Ellen Nakashima contributed to this report.