Special counsel Robert Mueller is looking into whether former national security adviser and Trump campaign aide Michael Flynn played any role in an effort to get Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE’s emails from Russian hackers, The Wall Street Journal reported Friday.

Mueller, who was appointed by the Justice Department to probe Russian efforts to meddle in last year’s presidential campaign, is also beginning to subpoena witnesses for grand jury testimony, according to NBC News.

Then-candidate Donald Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE publicly asked for Moscow to release any of Clinton’s emails it had during the 2016 campaign.

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Republican activist Peter W. Smith, who allegedly led an operation hoping to obtain Clinton’s deleted emails, portrayed Flynn as “an ally in those efforts and implied that other senior Trump campaign officials were coordinating with him” in correspondence and conversations with colleagues, according to the Journal. Flynn’s consulting firm and his son were also reportedly mentioned in the same correspondence.

Smith said he made contact with five different groups of hackers who claimed to have obtained Clinton’s emails, two of which Smith believed were Russian groups.

The Democratic presidential nominee faced months of criticism for the private "homebrew" email server she maintained while secretary of State, and emails allegedly deleted from that server were the subject of intense speculation.

Now federal investigators working for Mueller are examining whether Flynn, who was a senior Trump campaign adviser at the time, or his son were involved in any way in the pursuit of the emails. They are also working to determine whether Smith or anyone else paid hackers for Clinton’s emails, according to the report.

Lawyers for Flynn and his son both declined to comment to the Journal, as did a spokesperson for the special counsel.

Flynn resigned as national security adviser earlier this year after news broke that he had misled Vice President Pence — as well as the public — about the subjects discussed in his meetings with Russian officials.

The report notes that the inquiries suggest that Smith’s pursuit of the emails may be significant to the broader probe, which is examining any possible collusion between members of Trump's campaign or administration and Moscow.

Mueller was authorized to investigate “any links and/or coordination between the Russian government and individuals associated” with the Trump campaign.