WASHINGTON — A former adviser to Donald Trump’s presidential campaign pleaded guilty earlier this month to making false statements and “material omissions” to the FBI about numerous communications he had with allies of the Russian government, according to a criminal statement unsealed on Monday.

George Papadopoulos, who served as a foreign policy adviser to the Trump campaign, lied to federal investigators about his interactions with a professor linked to the Kremlin, according to the statement of offense filed with the U.S. District Court for the District of Columbia. The professor told Papadopoulos that the Russians had "dirt" on then-candidate Hillary Clinton in the form of "thousands of emails."

In a Jan. 27, 2017, interview with the FBI, Papadopoulos claimed that his interactions with the professor occurred before he joined the campaign. He also said the professor was a "nothing" and "just a guy talk(ing) up his connections" to high-level officials in the Russian government. The professor is not named in the court filing.

But "in truth and in fact," Papadopoulos knew the professor "had substantial connections to Russian government officials," the criminal statement says, and his contacts with the professor occurred while he was working for the Trump campaign.

The unnamed professor also introduced Papadopoulos to other Russian contacts, including a woman whom Papadopoulos described as Russian President Vladimir Putin’s niece and a Russian national connected to that government's ministry of foreign affairs. The court filing says Papadopoulos later learned the Russian woman was not related to Putin.

Papadopoulos was first interviewed by the FBI shortly after Trump's inauguration and again in February, both at a time when James Comey was still director of the FBI. Trump fired Comey in May, a move that eventually led to the appointment of Robert Mueller as special prosecutor charged with investigating Russia's involvement in the 2016 election.

Papadopolous pleaded guilty to the charges on Oct. 5, but it was kept secret until Monday because he was working with the government as a "proactive cooperator” in the Russia probe, according to the government's motion to seal his arrest and plea records. Additional charges in the Mueller investigation were also filed Monday against two major players in Trump’s campaign: former campaign chairman Paul Manafort and his business associate and campaign aide Rick Gates.

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Papadopoulos tried repeatedly to set up meetings between the Russian government and Trump campaign officials, with the help of the professor and other well-connected Russian contacts, the court filing shows.

"Have some interesting messages coming in from Moscow about a trip when the time is right," Papadopoulos wrote in an April 27, 2016, email to a senior Trump campaign policy adviser. In another email that same day, he told a second high-level campaign official that Putin was interested in hosting Trump.

"Have been receiving a lot of calls over the last month about Putin wanting to host him and the team when the time is right," he wrote in that second message.

During the summer of 2016, Papadopoulos started pursuing an "off-the-record" meeting between the campaign and high-level Russian officials.

"I am willing to make the trip off the record if it's in the interest of Mr. Trump and the campaign to meet specific people," he wrote in a June 19, 2016, email to a high-level Trump campaign official, who is not identified by name in the government's filing.

One month after Trump officially became the GOP nominee at the Republican National Convention, an unnamed campaign supervisor appeared to give Papadopoulos the green light for a trip to Moscow. "I would encourage you" and another foreign policy campaign adviser to "make the trip" if it's feasible, Papadopoulos' supervisor said.

As Trump's campaign operatives considered Papadopoulos' efforts to arrange a meeting with the Russians, they appeared to rule out any direct meeting involving Trump himself.

"We need someone to communicate that DT is not doing these trips," one unidentified campaign official wrote in an email to another, according to a footnote in the government's filing against Papadopoulos. "It should be someone low level so as not to send any signal."

The proposed trip did not take place, according to the court filing.

White House spokeswoman Sarah Huckabee Sanders downplayed Papadopoulos' role in Trump's 2016 campaign.

"He was a volunteer member of an advisory council that literally met one time," Sanders told reporters at the White House briefing on Monday. Any contacts he made with the Russians were done "on his own," she said, and he was rebuffed when he tried to arrange meetings between campaign officials and Russian officials.

Papadopoulos joined the Trump campaign in early March, and he met the professor on or about March 14, 2016, the court filing states. The professor "only took interest" in Papadopoulos because of his role in the campaign, and the professor told Papadopoulos about the "thousands of emails" on or about April 26, 2016, when he had been on board with the Trump operation for more than a month.

That was about two months before the Democratic National Committee revealed that its computer servers had been hacked; a security firm the DNC hired to investigate the breach concluded that Russia was behind the cyberattack.

Papadopoulos’ attorneys, Robert W. Stanley and Thomas M. Breen, said they could not comment on his case.

“It is in the best interest of our client, George Papadopoulos, that we refrain from commenting on George’s case,” the two criminal defense attorneys said in a joint statement. “We will have the opportunity to comment on George’s involvement when called upon by the court at a later date. We look forward to telling all of the details of George’s story at that time.”

Papadopoulos was arrested over the summer and has agreed to cooperate with the federal probe into Russia's efforts to sway the outcome of the 2016 presidential election. The court filing made public on Monday says Papadopoulos was arrested at Dulles Airport on July 27, 2017. After he was arrested, the government filed a motion to seal Papadopoulos' guilty plea because it might tip off other potential targets of the probe and give them an opportunity to destroy documents and other evidence.

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