The White House and an attorney for Flynn did not respond to a request for comment.

Mueller, whose office declined to comment, is investigating whether the Trump campaign had improper contacts with the Russians. Trump has denied any wrongdoing, and has repeatedly called the probe a "witch hunt."

Flynn pleaded guilty in late 2017 to lying to investigators about contacts he had with Russian government officials, and has been cooperating with special counsel Robert Mueller since then. He is due to be sentenced in December.

That operative, Peter Smith, told associates during the 2016 presidential campaign that he was using Flynn's connections to help him track down the emails, the paper reported. An investment banker and Republican fundraiser, Smith died in an apparent suicide in 2017.

A longtime Republican operative who spent the final months of his life seeking out thousands of Hillary Clinton's deleted emails developed a relationship with President Donald Trump 's former national security advisor Lt. Gen. Michael Flynn as early as 2015, The Wall Street Journal reported Wednesday, citing people familiar with the matter.

Michael Flynn, former national security advisor to U.S. President Donald Trump, walks out of the E. Barrett Prettyman Federal Courthouse after a status hearing July 10, 2018 in Washington, DC.

Mueller's prosecutors are interested in whether anyone in Trump's orbit assisted Smith in his search for Clinton's emails, according to the Journal. One witness familiar with his search was called before a grand jury, the paper reported. And a grand jury subpoena sought out documents related to Smith's hunt.

The quest for Clinton's emails drew Smith to the anti-secrecy organization Wikileaks, an anti-secrecy group which U.S. investigators have said was used by Russian intelligence services to disrupt the presidential election. The Journal said that in a December 2016 email, Smith told supporters that he had found "multiple individuals" who possessed Clinton's emails, and directed at least one of them to provide those emails to Wikileaks.

The nature of Smith's death has raised some suspicion.

He was found dead in a hotel in Minnesota days after telling The Wall Street Journal in an interview that he had been seeking Clinton's emails from Russian hackers. A suicide note found near him said: "NO FOUL PLAY WHATSOEVER."

The reporter who interviewed Smith later wrote in a tweet that he saw "no indication that he was ill or planning to take his own life." In June, it was made public via a court filing that Kathryn Rakoczy, an attorney known for her work in violent crimes cases, had joined the Mueller probe.

Trump himself repeatedly suggested during the 2016 election that the emails Clinton's attorneys deleted from her private server contained incriminating information. In a press conference in July of that year, the president appealed to Russian hackers to make the emails public.

"Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing," Trump said at the time.

This summer, Mueller's investigators obtained an indictment against 12 Russian hackers that accused them of attempting to break into Clinton's personal server for the first time that same day.

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