Special counsel Robert Mueller has reportedly impaneled a grand jury in Washington as part of the ongoing investigation into Russia's interference in the 2016 presidential election, The Wall Street Journal reported Thursday.

Such a move would signal that the investigation is growing and entering a new phase. The special counsel is looking into Russia's efforts to influence the election and any ties between President Trump's campaign and Moscow.

Mueller’s spokesman declined to comment to the newspaper, while White House special counsel Ty Cobb said he was unaware of the new development.

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“Grand jury matters are typically secret. The White House favors anything that accelerates the conclusion of his work fairly," Cobb said in a statement. "The White House is committed to fully cooperating with Mr. Mueller."

The grand jury, located in Washington, D.C., follows a separate grand jury previously convened in Alexandria, Va., by federal prosecutors investigating former national security adviser Michael Flynn.

“Former FBI Director Jim Comey said three times the president is not under investigation and we have no reason to believe that has changed,” said White House press secretary Sarah Huckabee Sanders when asked about the grand jury.

The news follows efforts from some in Congress to protect Mueller from Trump and his administration’s ability to fire him.

Sen. Lindsey Graham Lindsey Olin GrahamSenate Republicans face tough decision on replacing Ginsburg Democratic senator calls for eliminating filibuster, expanding Supreme Court if GOP fills vacancy What Senate Republicans have said about election-year Supreme Court vacancies MORE (R-S.C.) and Cory Booker (D-N.J.) introduced a bill on Monday that would require the Justice Department to seek a request from a federal judge before firing Mueller or any other special counsel.

"Our bill allows judicial review of any decision to terminate a special counsel to make sure it’s done for the reasons cited in the regulation rather than political motivation. I think this will serve the country well," Graham said in a statement.

Booker added that special counsel should never be subjected to "interference or intimidation because of where an investigation takes them."

Mueller continues to expand the team investigating Russia’s interference in the 2016 presidential election with the addition of former Justice Department official Greg Andres, who became the 16th lawyer to join Mueller’s team. Andres previously worked in the Justice Department’s criminal division.

Trump has ratcheted up attacks on Mueller as the investigation proceeds. In an interview with The New York Times in July, Trump warned Mueller against investigating his family’s finances.

“I think that’s a violation. Look, this is about Russia,” Trump told The Times.

Trump also argued that Mueller has hired “Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE supporters” and is good friends with Comey, who Trump fired in early May. Comey had been leading the FBI investigation into Russian election interference and Trump campaign ties with Moscow before Mueller.

“Well he’s very, very good friends with Comey. Which is very bothersome,” Trump told Fox News. “We’re going to have to see. But there has been no obstruction, there has been no collusion. There has been leaking by Comey. And virtually everyone agrees on that.

“The people who have been hired are all Hillary Clinton supporters,” he said. “Some of them worked for Hillary Clinton. I mean, the whole thing is ridiculous if you want to know the truth from that standpoint."

Trump’s own legal team has reportedly been looking into Mueller’s team for potential conflicts of interest to potentially undermine or discredit the investigation.

The president has called the investigation into Russia's election interference and his campaign's ties to Moscow a "witch hunt."

“Special Counsel Mueller impaneling a grand jury is further confirmation that this is a serious investigation and not a ‘witch hunt,’” Democratic National Committee CEO Jess O’Connell said in a statement.

- Updated at 6:09 p.m.