Robert Mueller, the special counsel overseeing the Russia investigation, has impaneled a grand jury in Washington, The Wall Street Journal reported Thursday.

The move means the probe is intensifying and could stretch "for months," according to the newspaper. Impaneling a grand jury suggests Mueller "believes he will need to subpoena records and take testimony from witnesses," the Journal said.

It does not necessarily mean he will bring charges against Trump allies.

Former FBI Director Mueller is investigating Russian efforts to influence the 2016 election and whether the Trump campaign colluded with the Kremlin. The investigation has dogged and frustrated President Donald Trump during his first six months in office.

The president has repeatedly called it a "witch hunt" and denied any collusion with Moscow. He and his allies have also been critical of Mueller.

The Justice Department appointed Mueller after Trump fired former FBI chief James Comey.

White House press secretary Sarah Huckabee Sanders issued a statement responding to the grand jury report:

Ty Cobb, special counsel to the president, said he wasn't aware that Mr. Mueller had started using a new grand jury. "Grand jury matters are typically secret," Mr. 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." Former FBI Director Jim Comey said three times the President is not under investigation and we have no reason to believe that has changed.

Trump lawyer John Dowd told NBC News the president's legal team has been "cooperating with Bob Mueller and his staff since the first of June because we're trying to get this thing over and done with."

Separately, CNN reported that Mueller's team has looked toward Trump and his associates' possible financial relationships with Russia. It "could offer a more concrete path toward potential prosecution" than the question of collusion with Moscow, according to the news network.

Trump previously told The New York Times he considers Mueller looking at his finances a "red line" in the investigation.

Concerns have grown that Trump could try to find a mechanism to remove Mueller. Bipartisan senators even put forth new legislation to legally protect the special counsel.