One of President Trump's former campaign advisers has pleaded guilty to lying to FBI agents who are investigating possible collusion between the Trump campaign and the Russian government, the Department of Justice said Monday.

George Papadopoulos, the 30-year-old foreign policy adviser, was arrested in July and pleaded guilty earlier this month, according to court filings unsealed on Monday. The Justice Department unsealed the documents shortly after it announced charges against two other higher-ranking Trump campaign officials — Paul Manafort and Rick Gates.

The charges stem from special counsel Robert Mueller's investigation.

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Prosecutors charged Papadopoulos with lying to investigators about his conversations with a foreign professor who told him that Russians had thousands of emails containing "dirt" on Democratic presidential nominee 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.

He allegedly lied about his contact with a Russian woman whom the Justice Department says he worked with "in an effort to arrange a meeting between the campaign and Russian government officials." Papadopoulos initially identified the woman as Russian President Vladimir Putin's niece, but later found out she was not related to Putin.

Papadopoulos allegedly told the FBI that those conversations happened before he joined the Trump campaign, statements rebutted by the Justice Department's timeline.

Charging documents paint a picture of Papadopolous as eager to leverage his relationships with the professor and the Russian woman as part of his role with the Trump campaign. He told a group of fellow foreign policy advisers and Trump himself during a spring 2016 meeting that he could use his connections to broker a meeting between Trump and Putin, authorities said.

To that end, Papadopoulos allegedly emailed a Russian with connections to the Russian Ministry of Foreign Affairs, forwarding information from the correspondences to a top campaign official not identified in the documents. On numerous occasions, he tried to negotiate a meeting with either Trump or campaign officials and "members of President Putin's office" as well as the Russian Ministry of Foreign Affairs, officials said.

Those meetings never occurred, but officials in the Trump campaign appeared to encourage his efforts.

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When Papadopolous told one campaign official about his early attempts to work with the professor and the Russian woman to orchestrate a meeting, the supervisor told him in an email: "Great work."

The campaign supervisor also encouraged him to take an "off the record" meeting with Russian officials "if it is feasible," according to the emails

A second campaign official at one point said Trump himself shouldn't take any trip to meet with the Russians; instead, "it should be someone low level in the campaign so as not to send any signal."

While Papadopolous disclosed the meeting with the professor to FBI agents during a January interview, he told them that they spoke before he joined Trump's campaign, authorities said, adding that he sought to play down his own interest in the professor's claims because he thought the man was "BS'ing" him.

According to authorities, Papadopoulos told them about the professor: "This isn't like he's messaging me while I'm in April with Trump."

"This was a year ago, this was before I even got with Trump."

But the FBI found that the meetings happened after Papadopoulos had been told he would join the campaign and that the professor's interest in him only stemmed from his position with Trump's campaign.

He also allegedly made a similar claim about his relationship with the Russian woman, arguing that he had only exchanged pleasantries with her when he had in fact emailed her about arranging meetings between the Russians and Trump representatives.

Following his interview with the FBI, authorities say Papadopoulos deactivated the Facebook account that had information about his conversations with both the professor and the Russian with connections to the Ministry of Foreign Affairs. He also changed his cell phone number.

Papadopoulos's name emerged in media reports back in August, after The Washington Post uncovered internal emails he sent offering to broker a meeting with the campaign and the Russians.

The Post reported at the time that Trump aides didn't warm to the idea, with some top officials raising concerns that a meeting could either violate federal law or reflect poorly on the campaign.

The former Trump aide faces up to five years in prison, a fine of $250,000 and supervised release as part of a plea deal with the government. But the agreement notes that prosecutors estimate he'll serve no more than six months in jail and face a fine between $500 to $9,500 as long as he isn't found to have committed any additional crimes.

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His plea deal could open the door for further cooperation with the special counsel's investigation, something noted on Twitter by former U.S. District Attorney Preet Bharara, a vocal Trump critic who was fired by the president.

Special Counsel Mueller appears to have a cooperating witness, George Papadopoulos. That is significant. Time will tell how significant. — Preet Bharara (@PreetBharara) October 30, 2017

This report was updated at 12:37 a.m.