Special Counsel Robert's Mueller's investigation of Russia's efforts to meddle in the 2016 election now includes inquiries into a controversial, unverified opposition research dossier on President Trump compiled by a former British intelligence officer, according to a Reuters report.

Reuters reported Wednesday that Mueller's team of investigators has assumed control of multiple inquiries into allegations that the Kremlin sought to interfere in the election to sway it in President Trump's favor.

The U.S. intelligence community said in a report made public in January that they took the allegations made in the dossier seriously. It identifies several Russian businessmen and others whom the intelligence community believes are Russian operatives.

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Trump was made aware of the dossier weeks before taking office when former FBI Director James Comey briefed him on the matter. The president has denied allegations of collusion between his campaign and Moscow, and has called Mueller's investigation "a witch hunt."

The Russian government has also denied that it meddled in the election.

The opposition research dossier was originally paid for by an anti-Trump GOP group. But after the real estate mogul secured the Republican nomination in July 2016, a pro-Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE group began paying for work on the dossier to continue.

The revelation that Mueller's team has taken over FBI inquiries into the dossier authored by former MI6 officer Christopher Steele came the same day that the top lawmakers on the Senate Intelligence Committee said that their investigation into Russian election meddling was expanding.

Sen. Richard Burr Richard Mauze BurrRep. Mark Walker says he's been contacted about Liberty University vacancy Overnight Defense: Trump rejects major cut to military health care | Senate report says Trump campaign's Russia contacts posed 'grave' threat Senate report describes closer ties between 2016 Trump campaign, Russia MORE (R-N.C.), the committee's chairman, said that the panel had attempted to interview Steele regarding the dossier, but that he has yet to agree to talk to congressional investigators.

“The committee cannot really decide the credibility of the dossier without understanding things like who paid for it, who are your sources and sub-sources,” Burr said at a news conference.