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WASHINGTON (CBSMiami) — Special Counsel Robert Mueller has impaneled a grand jury in Washington, D.C. in a move that signals that the investigation into Russian meddling in the 2016 election for president is gaining steam, the Wall Street Journal reported.

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FBI Director Robert Mueller testifies during a hearing before the House Judiciary Committee July 26, 2007 on Capitol Hill in Washington, DC. The hearing was held to examine whether the FBI have misused their power. (Photo by Alex Wong/Getty Images)

Grand juries allow prosecutors to put witnesses under oath. According to the Wall Street Journal, the grand jury was convened in Washington within the last few weeks. Another grand jury in Virginia, investigating former national security adviser Michael Flynn has converged under Mueller, the Journal said.

The probe centers around whether President Donald Trump, his campaign or associates colluded with Russia to win the election. The president has since denied any wrong-doing and has called the investigation a “witch hunt.”

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“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.”

President Trump has suggested he might attempt to fire Mueller, the 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.

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White House Press Secretary Sarah Huckabee Sanders noted that on three different occasions, former FBI Director James Comey said the president was not under investigation and “we have no reason to believe that has changed.”