An email sent by George Papadopoulos last year provides the first hard evidence that the former Trump campaign adviser was friends with Sergei Millian, an alleged source for some of the most salacious claims in the infamous Steele dossier.

The Washington Post reported Tuesday night that Papadopoulos sent an email to Trump aide Boris Epshteyn last September seeking to set up a meeting with Millian, who he reportedly described as a friend. Epshteyn told The Post that the meeting did not occur.

Papadopoulos, a 30-year-old energy consultant, is in the news this week because it was revealed that he pleaded guilty earlier this month to lying to the FBI about contacts he had with Russians just after he joined the Trump team in March 2016. Documents showing Papadopoulos’ July 27 arrest and his Oct. 3 plea deal were released on Monday by the Office of Special Counsel Robert Mueller, which is investigating possible collusion between the Trump campaign and Russian government. (RELATED: How Did George Papadopoulos End Up On The Trump Campaign?)

The link between Papadopoulos and Millian is significant because it could explain how information from someone in the Trump campaign ended up in the dossier, which was financed by the Clinton campaign and Democratic National Committee. The jaw-dropping document cites unnamed Trump campaign associates as the ultimate sources for the allegations about collusion with the Kremlin.

The dossier appears to be a major basis of the Mueller investigation, which spun out of an FBI probe that began last July, shortly after FBI agents were briefed by Christopher Steele, the former MI6 agent who wrote the dossier.

Mueller’s investigators recently traveled to London to interview Steele, who operates a firm called Orbis Business Intelligence. The Daily Caller has also been told by a source familiar with the investigation that questions have been asked about Millian in witness interviews.

The fact that Papadopoulos and Millian are friends does not necessarily mean that the Trump campaign adviser is a primary source for the dossier. And if he was, it wouldn’t necessarily prove that the allegations attributed to Millian in the document are true.

It could be the case that if Papadopoulos provided insight about the campaign to Millian, the information could have been inaccurate or embellished. And if Millian relayed that information to the intermediary who provided it to Steele, that information could also have been inaccurate or embellished.

And both Papadopoulos and Millian, whose real name is Siarhei Kukuts, appear to have overstated their accomplishments and influence in the past. Papadopoulos has lied about parts of his resume, and Millian has been described by one associate as a “hustler” and “self-promoter.” (RELATED: Alleged Dossier Source Has Overstated His Business Connections)

Neither Millian nor Papadopoulos responded to multiple emails and phone calls seeking comment, but some of those questions could soon be answered. The House Intelligence Committee has recently subpoenaed the FBI and Justice Department for information that could shed light on some of the sourcing for the dossier. The House panel also wants the agencies to explain whether they vetted the Steele document before using it as part of the collusion probe.

Papadopoulos’ attorney also did not respond to a request for comment.

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Millian, the chairman of the Russian-American Chamber of Commerce, was identified by The Wall Street Journal and ABC News back in January as “Source D” and “Source E” in the dossier, which Steele authored from June 20, 2016 to Dec. 13.

Steele cited “Source D” and “Source E” for several of the most serious allegations contained in the 35-page document, which has been used by the FBI as part of its investigation into possible collusion between the Russian government and Trump campaign. (RELATED: The Source Of Salacious Claims In The Dossier Has Been Identified)

Steele cited “Source D” in his June 20 memo as claiming that Russian intelligence services had been feeding Trump and his team with information about his political opponents, including Hillary Clinton.

The intelligence flow had been “very helpful,” the source claimed.

“Source D” also claimed to be present at a hotel in Moscow where Trump hired prostitutes. The dossier alleges that Kremlin operatives had recorded Trump during the hotel stay and has used the footage to blackmail the former real estate mogul.

And in a memo in July, “Source E” claimed that “there was a well-developed conspiracy of co-operation” between the Trump campaign and Russian leadership.

The source said that the operation was managed in the Trump campaign by then-campaign manager Paul Manafort. He used foreign policy adviser Carter Page and other intermediaries to carry out the effort, the source said.

Trump and members of his campaign have denied allegations in the dossier, calling it a “hoax” and “fake news.” Page has denied knowing Manafort or communicating with him. He also told TheDC recently that he does not know Millian. Manafort, who was indicted earlier this week by Special Counsel Robert Mueller on money laundering and tax charges, has also denied knowing Page.

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Though Millian, an American citizen born in Belarus, was not a member of the Trump campaign, he has claimed in the past to have close ties to members of the team. He has met the real estate mogul in the past. In 2007, he posed for a photo with Trump at a horse track in Florida. He has also claimed to have worked as a broker for the Trump Hollywood condominium.

In an interview last April with RIA News, Millian claimed to have been in contact with Trump associates “a few days” earlier.

Millian also believed at one point during the campaign that he was close to being named an adviser to Trump, a source close to him has told TheDC.

The source, who spoke on condition of anonymity because of the controversy surrounding Millian, said that they knew of no contacts between Millian and the Trump campaign and that they believed he was “living in fairy-tale land” if he expected to join the Trump team.

Millian expressed concern to his friend that his chances of becoming a Trump adviser were put in jeopardy because of an interview he had just conducted with ABC News.

In that July 2016 interview, Millian portrayed himself as a close business associate of Trump’s real estate company. He also said that Trump liked Russia because “he likes beautiful Russian ladies.”

Millian denied in the interview that he was involved with Russian intelligence services, but at the same time claimed to be “one of those very few people who have insider knowledge of Kremlin politics.”

He said that his “insider information and connections to Russia” could translate “to great benefits for the American administration.”

As for what information Papadopoulos could have provided Millian, were he a source for his friend, the Mueller team’s statement of offense against the young campaign adviser matches up with some of the allegations attributed to Millian in the dossier.

According to the document, Papadopoulos claimed that a professor with purported ties to the Russian government offered told him that the Russian government had obtained “thousands” of stolen Hillary Clinton emails.

Papadopoulos told federal investigators that a London-based professor since identified as Joseph Mifsud told him during an April 26, 2016 meeting in London that he had been told of the “dirt” by Russian government officials during a meeting in Moscow.

A month prior to that meeting, Russian hackers stole emails from Clinton campaign chairman John Podesta. They were published by WikiLeaks in October.

Whether Papadopoulos communicated that information to the Trump campaign or to Millian is unclear.

The day after the meeting with Mifsud, Papadopoulos sent an email to an unidentified campaign senior adviser saying that had “some interesting messages coming in from Moscow about a trip when the time is right.”

Mifsud has disputed Papadopoulos’ claims. On Tuesday he told a British newspaper that he does not have ties to the Russian government and that he knew nothing about Clinton’s emails. (RELATED: Mystery Professor Who Met With Trump Adviser Denies Allegation He Had ‘Dirt’ On Hillary)

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