(CBS News) -- Special Counsel Robert Mueller has impaneled a grand jury in Washington, D.C., a clear sign that the investigation into Russian interference in the 2016 election is ramping up, the Wall Street Journal reports.
Meanwhile, Reuters reported grand jury subpoenas have been issued in connection with the June 2016 meeting between Donald Trump Jr., a Russian lawyer and others.
The special counsel's office did not return CBS News' request for comment.
White House press secretary Sarah Sanders said that former FBI Director James Comey had "said three times that the president is not under investigation, and we have no reason to believe that has changed."
Grand juries give prosecutors the ability to put witnesses under oath if there is evidence of a crime. Mueller and his expanding team will also have the power to seek indictments and subpoena documents. According to the Journal, the grand jury was convened in Washington several weeks ago. Another grand jury in Virginia had been investigating former national security adviser Michael Flynn, but that probe has been taken over by Mueller, the Journal reported.
"This is yet a further sign that there is a long-term, large-scale series of prosecutions being contemplated and being pursued by the special counsel," Stephen I. Vladeck, a law professor at the University of Texas, told the paper. "If there was already a grand jury in Alexandria looking at Flynn, there would be no need to reinvent the wheel for the same guy. This suggests that the investigation is bigger and wider than Flynn, perhaps substantially so."
The existence of the grand jury does not mean that Mueller will necessarily bring charges, former federal prosecutor Thomas Zeno told the Journal, but it also shows he is "very serious."
"He wouldn't be doing this if it were winding down," Zeno said.
President Trump has repeatedly called the investigation into Russian interference, and the alleged connections between his campaign and Russian figures, a "witch hunt." He has also suggested that he might attempt to fire Mueller, a former FBI director whose investigation falls under the purview of the Department of Justice. In response, Congress is currently considering bills that would make it harder for Trump to fire Mueller.
Deputy Attorney General Rod Rosenstein appointed Mueller special counsel in May, and Mueller was given a broad mandate to investigate and prosecute any potential crimes he uncovered. The appointment came after Mr. Trump fired then-FBI director James Comey and Attorney General Jeff Sessions recused himself from the investigation, delegating oversight of the probe to Rosenstein.
Ty Cobb, a member of Mr. Trump's legal team, said he was unaware of the existence of the grand jury until he was notified by the Journal, noting "grand jury matters are typically kept secret."
"The White House favors anything that accelerates the conclusion of his work fairly ... The White House is committed to fully cooperating with Mr. Mueller," Cobb said.
