WASHINGTON — Special counsel Robert Mueller is reportedly tapping a federal grand jury in Washington to advance the federal investigation into Russia's interference in the presidential election, a sign the probe is intensifying and could go on for months or years.

The move would give Mueller, a former FBI director, broad authority to subpoena documents and compel witnesses to testify under oath.

Mueller has been investigating possible collusion between Trump associates and Russians who sought to influence last year's election by hacking Democrats. The probe's expansion is also virtually sure to incense President Trump, who is already furious with the Russia investigation he calls a "witch hunt."

The Wall Street Journal, which broke the story Thursday, reported that the grand jury began working in recent weeks. The panel has issued subpoenas relating to the controversial June 2016 meeting between the president's son, Donald Trump Jr., and a Russian lawyer, Reuters reported.

Last month, Trump Jr. released emails showing he arranged the meeting in the hopes of obtaining damaging information about his father's election opponent Hillary Clinton – even after he was told it would come from the Russian government.

President Trump's son-in-law and adviser Jared Kushner, along with then-campaign chairman Paul Manafort, were also in the meeting with Russian lawyer Natalia Veselnitskaya, and others with Russian ties.

Josh Stueve, a spokesman for Mueller, declined to comment on the reports.

Yet legal analysts characterized Mueller's use of a grand jury as a serious escalation of an inquiry that he began overseeing in May.

“This suggests that there is evidence that a crime may have been committed and there is a need to apply the legal tools a grand jury can bring to bear,’’ said Jimmy Gurule, a former assistant attorney general in the administration of President George H.W. Bush.

“With a grand jury, Mueller can compel witnesses to testify and collect documents that are central to the investigation.’’

Through witness testimony and documents, Gurule said, prosecutors are able to more "fully define the contours and the scope" of the activity being examined.

"This is not a step to be taken lightly," he said. “This is a significant action."

If Mueller is using a grand jury, said Jens David Ohlin, a vice dean and professor at Cornell Law School and an expert on criminal law, "this suggests the investigation will end with indictments."

While grand juries hear evidence and sometimes decline to hand up indictments, it now looks unlikely that Mueller will "simply submit a report to Congress and allow the political process to digest his findings," Ohlin said. "This is a criminal investigation in the fullest sense of the term."

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The news appeared to take the Trump team by surprise.

John Dowd, Trump's lead outside lawyer, said Thursday that he was not aware of Mueller's reported move to impanel a new grand jury in D.C., but said he was confident that the president was not a target of prosecutors.

"With respect to reports of a federal grand jury, I have no reason to believe that President Trump is being investigated," Dowd said.

At a rally in West Virginia Thursday night, Trump did not address the latest Russia investigation developments. But he once again called the Russian investigation a hoax and blamed Democratic lawmakers for perpetuating it.

"They're trying to cheat you out of the leadership you want with a fake story that is demeaning to all of us and most importantly demeaning to our country and our Constitution," he said. "I just hope the final determination is a truly honest one, which is what the millions of people who gave us our big win in November deserve."

The White House also pushed back on the idea Trump was being investigated. Former FBI Director James Comey "said three times the President is not under investigation and we have no reason to believe that has changed," it said in a statement provided by spokeswoman Sarah Sanders.

Yet Trump's own responses to the Russia investigation are also under scrutiny.

Trump in May abruptly fired Comey, who in subsequent testimony, alleged that the president tried to get him to drop an investigation into former national security adviser Michael Flynn, who was fired in February after revelations he lied to administration officials about his contacts with Russian officials.

Trump has also recently stepped up criticism of his own attorney general, Jeff Sessions for recusing himself in the Russia probe – another move that has raised questions about whether Trump was trying to obstruct or wrest control over Mueller's probe.

Earlier this week, Trump's actions were in the spotlight again, after a Washington Post report that Trump personally dictated a statement from Trump Jr. that later proved to be inaccurate and misleading.

The White House acknowledged Tuesday that Trump "weighed in" on the disputed statement, which appeared to contradict statements made by Trump's own lawyers that the president was not involved in the drafting process. The initial July 8 statement provided by Trump Jr. said the meeting primarily had to do with the adoption of Russian babies and was not a campaign issue.

On Thursday, after reports of the grand jury surfaced, the White House said in a statement provided by spokeswoman Sarah Sanders that Comey "said three times the President is not under investigation and we have no reason to believe that has changed."

While Ty Cobb, special counsel to the president, also said he did not know that Mueller had called a grand jury, he stressed the White House would comply with the federal investigation.

"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."

Yet Trump has objected to previous reports that Mueller may expand his probe to include his personal finances, a move Trump has said would exceed Mueller's authority. In a July 19 interview with The New York Times, Trump said, "I have done nothing wrong. A special counsel should never have been appointed in this case."

Legal experts said the use of a grand jury shows that the federal Russia probe has now gone far beyond the investigation of Flynn, Trump’s former national security adviser, who misled colleagues about his discussions with the Russian ambassador to the U.S., Sergey Kislyak, before Trump's inauguration.

A grand jury had already been investigating the activities of Flynn, a retired Army lieutenant general.

"Impaneling a new grand jury in D.C. suggests that Mueller has a probe that goes beyond Flynn’s conduct, and that could go on for many months,” said John Wood, a former U.S. attorney and former chief of staff at the Department of Homeland Security.

“If Mueller were close to wrapping up his investigation, he likely would have used the grand jury that was already impaneled in Alexandria, Virginia, which was investigating Michael Flynn," Wood continued. "The fact that Mueller recently recruited former prosecutor Greg Andres from a leading Wall Street law firm further confirms that his investigation will be broad and will last for some significant amount of time.”

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Meanwhile, Democrats in Congress welcomed news of the development.

“You don’t impanel a grand jury if you only have smoke. Mueller must be seeing fire. (Trump) and/or his associates in bigly trouble," tweeted Rep. Ted Lieu, D-Calif. And Sen. Patrick Leahy, D-Vt. added: "Americans deserve answers. I hope this brings us one step closer."

The top Democrat on the House Intelligence Committee, which is also investigating Russia's election meddling, said news that Mueller has impaneled a grand jury "suggests his work is proceeding."

"All the more (important) that Congress protect his independence," tweeted Rep. Adam Schiff, D-Calif.

Earlier Tuesday, senators from both parties took steps to ensure that Mueller stays special counsel – despite Trump's recent digs at the former FBI director whom he has accused of having unspecified conflicts of interest.

Two bills introduced Thursday, right as the Senate prepared to leave for its August break, are designed to protect special counsels from political interference.

Outside watchdog groups such as Common Cause also joined in warning Thursday against possible retaliation by Trump.

“The impaneling of a grand jury to investigate Russian interference in the 2016 presidential election hammers home the imperative that the Trump administration must not interfere with Robert Mueller’s investigation or that of the grand jury,” said Karen Hobert Flynn, president of Common Cause.

“Regardless of whether the President wants to continue to call Russian election interference ‘fake news,’ he and his administration must take this criminal investigation seriously and offer investigators their full cooperation. The president must not follow through with his threats to fire Robert Mueller, but if he does Congress must hold him and members of his administration accountable because no American, not even the President, is above the law.”

– Gregory Korte and Jessica Estepa contributed