The president's former foreign policy adviser George Papadopoulos secretly pleaded guilty Monday to lying about contacts he's had with Russians. (Matt McClain/The Washington Post via Getty Images)

Amid breaking news of criminal indictments against two top Trump campaign officials, the FBI detonated another bombshell Monday: President Donald Trump's former foreign policy adviser has secretly pleaded guilty to lying about contacts he's had with Russians during interviews with federal law enforcement officials.

As part of an agreement with the government, George Papadopoulos, 30, entered a plea Oct. 5 on charges he made "material false statements and material omissions" when FBI agents interviewed him in January, according to court documents unsealed Monday.

A conservative global-affairs academic living in London, Papadopoulos met with three Russian nationals just weeks after joining Trump's campaign and told Trump's advisers he could help open up back-channel communications between the candidate and Russian President Vladimir Putin, according to the court documents.

Although Trump has slammed the Russia investigation as a witch hunt, the FBI learned that Papadopoulos sent emails to top Trump campaign operatives about the possibility of direct talks between Trump and Putin. According to the documents, Papadopoulos told the campaign his contacts could help "arrange a meeting between the [Trump] Campaign and the Russian government" as well as deliver "dirt on Hillary Clinton," Trump's Democratic rival for the White House.

While the criminal indictments of Paul Manafort, Trump's former campaign manager, and Paul Gates, Manafort's longtime associate, grabbed the headlines Monday, legal experts say news of Papadopoulos' plea deal is significant: Court papers indicate Papadopoulos is cooperating with special prosecutor Robert Muller, and may have worn a wire to help Muller's team gather evidence.

At the same time, despite the serious nature of the charges – and the White House's description of Papadopoulos as a campaign volunteer with little influence – Mueller himself asked a federal judge to keep Papadopoulos' plea agreement under wraps to avoid compromising the investigation.

"[Papadopoulos] has indicated that he is willing to cooperate with the government in its ongoing investigation into Russian efforts to interfere in the 2016 presidential election," according to a motion Mueller filed and signed, asking the court to seal the file. "Public disclosure of the defendant's initial appearance, however, would significantly undermine his ability to serve as a proactive cooperator."

The law firm of Breen & Pugh, which is representing Papadopoulos, refused to discuss the extent of their client's involvement with Mueller. "We will have the opportunity to comment on George's involvement when called upon by the Court at a later date," according to a press release from his attorneys.

According to the government's summary of the case, Papadopoulos' contact with the Russians began shortly after he signed on with Trump's campaign in March 2016.

A London resident at the time, Papadopoulos' initial contact was with a London-based man authorities only identify as "the professor." The documents don't spell out how the two met; that connection, however, resulted in introductions to others identified in the documents as "the female Russian national" – presented to him as Putin's niece, though it was later determined she was not – and the Russian "Ministry of Foreign Affairs contact."

Papadopoulos "learned he would be an advisor to the Campaign in early March, and met the professor on or about March 14, 2016; the professor only took interest in [him] because of his status with the Campaign," according to the complaint. A few weeks later, the complaint states,

"the professor told [him] about the 'thousands of emails' written by Clinton which he purportedly had access to" and offered to make a connection.

Over the course of the spring and summer, Papadopoulos and the professor connected with the female Russian national and the Ministry of Foreign Affairs contact, according to the complaint.

Although Papadopoulos insisted to the FBI the meetings didn't yield significant information, the agents later learned that Papadopoulos had reported the contacts to the upper echelons of the Trump campaign. Papadopoulos reached out to top campaign operatives via email about his Russian contacts, urging them to take advantage of the situation.

When he gave a voluntary interview to the FBI in January 2017, Papadopoulos initially suggested his contact with the Russians didn't amount to much.

Papadopoulos denied that he believed the contacts would lead to anything, insisting that the professor "was just a guy talking up connections or something."

A month later, the FBI again interviewed Papadopoulos – this time, with his lawyer present.

"During the course of the interview, [Papadopoulos] made numerous false statements and omitted material facts regarding the conduct and communications described above," according to the court documents. Specifically, a criminal complaint states, Papadopoulos "lied about the extent, timing, and nature of his communications with the professor, the female Russian national, and the Russian MFA Connection."

"In truth and in fact, however, [Papadopoulos] understood that the professor had substantial connections to Russian government officials ... and, over a period of months, [Papadopoulos] repeatedly sought to use the professor's Russian connections in an effort to arrange a meeting between the Campaign and Russian government officials," according to the complaint.

Since his arrest at Dulles International Airport outside Washington on July 27, Papadopoulos has cooperated with Mueller's team, according to the complaint. Prosecutors agreed to recommend between no prison time to six months under the plea agreement.

The Washington Post reported Papadopoulos is a 2009 graduate of DePaul University and was an adviser to Ben Carson's presidential campaign. He was the U.S. representative at the Geneva International Model United Nations in 2012.

At a press briefing Monday afternoon, White House press secretary Sarah Huckabee Sanders said Papadopoulos was a low-level campaign volunteer who had little influence on the campaign, even though Trump touted his involvement in an interview with The Washington Post editorial board.

"This individual was a member of a volunteer council that met one time over the course of a year," she told reporters. "He was part of a list that was read out in The Washington Post. I hardly call that some sort of regular adviser or, as you want to push, that he is like a senior member of the staff. He was not paid by the campaign. He was a volunteer on a council that met once."

Updated on Oct. 30, 2017 : This story has been updated with additional information.