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WASHINGTON — A grand jury has been impaneled by the special counsel investigating possible links between Russia and Donald Trump’s presidential campaign, The Wall Street Journal reported Thursday.

The paper reports that the grand jury is a new and significant development in Special Counsel Robert Mueller's probe into whether Trump or his top advisers colluded with Moscow to advance the Republican's presidential campaign in 2016.

NBC News has previously reported that before Mueller, the former FBI director now leading the probe, was appointed, federal prosecutors had convened grand juries as part of the investigations into former National Security Adviser Michael Flynn and Trump’s former campaign chairman Paul Manafort.

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John Dowd, one of the president’s private lawyers, told NBC News he was not aware that Mueller had started using a grand jury.

"We have 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," he said. "With respect to news of a federal grand jury being impaneled, we have no reason to believe President Trump is under investigation."

NBC News has not independently verified the Journal report.

The use of a grand jury is part of the normal process of a complex investigation and it is a tool not just for hearing testimony but for learning about and obtaining records from individuals or companies. It is a powerful tool prosecutors can use to seek indictments and subpoenas, and may be a signal Mueller's work is far from over.

Trump has denied that he or any members of his team colluding with Russia during the 2016 presidential campaign. Officials in Moscow have also denied playing a role in Trump's surprise victory last November.

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," Cobb said in a statement. "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."