Special counsel Robert Mueller is using a grand jury in Washington, D.C., in connection with his investigation into Russian efforts to influence the 2016 presidential election and possible collusion between Russia and top aides to the Trump campaign, a source with knowledge of the investigation confirms to NPR’s Peter Overby. The source did not want to be identified because of the sensitivity of the matter.

The Wall Street Journal first reported that Mueller, a former FBI director, was using a grand jury. The latest development signals the former FBI director’s investigation is “growing in intensity,” with the grand jury beginning work in recent weeks, the Journal reported.

A spokesman for Mueller declined to comment to NPR.

Ty Cobb, a recently appointed White House special counsel focused the on Russia probes, said in a statement provided by White House press secretary Sarah Huckabee Sanders that he wasn’t aware Mueller had started using a grand jury.

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

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President Trump has maintained there was no collusion between Russia and his campaign during the 2016 election. And Trump has, at times, cast doubt on the determination of the U.S. intelligence community that Russia was behind various efforts to interfere in the election, including the hacking of emails belonging to Democratically-aligned individuals and organizations and the strategic release of those emails at key points in the campaign last year.

But in a statement Wednesday announcing he had signed into law a bill imposing sanctions on Russia, Trump said he supported “making clear that America will not tolerate interference in our democratic process and that we will side with our allies and friends against Russian subversion and destabilization.”

Trump has repeatedly called Mueller’s probe a “witch hunt” and some media reports have indicated the president has looked into ways to undercut or even fire Mueller.

“With respect to the impaneling of the grand jury, we have no reason to believe that the President is under investigation,” Jay Sekulow, a lawyer on the president’s outside legal team, told NPR’s Tamara Keith.

The revelation that Mueller’s investigation is utilizing a grand jury comes as CNN reports investigators have zeroed in on “Trump and his associates’ financial ties to Russia as one of the most fertile avenues for moving their probe forward,” with sources telling them that the “web of financial ties could offer a more concrete path toward potential prosecution than the broader and murkier questions of collusion in the 2016 campaign.” That includes ongoing investigations into former White House national security adviser Michael Flynn and Trump’s former campaign chairman Paul Manafort.

But Mueller dipping into Trump’s own personal finances is exactly what the president argued to the New York Times last month would be a “violation” of the scope of investigation delineated at the time Mueller was appointed by the Justice Department.

Reuters is also reporting that grand jury subpoenas have been issued regarding Donald Trump Jr.’s June 2016 meeting with a Russian attorney who Trump Jr. was told would have incriminating information about Democratic presidential nominee Hillary Clinton.

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