A former Trump campaign adviser struck a cooperation agreement with Special Counsel Robert Mueller, secretly pleading guilty three weeks ago to lying to federal agents about his contacts with Kremlin-connected Russians.

The bombshell announcement about George Papadopoulos came shortly after indictments against former campaign chair Paul Manafort and Manafort associate Rick Gates were unsealed, alleging financial crimes that predated Trump's candidacy.

READ MORE | Manafort, Gates charged with conspiracy in Mueller investigation

"Through his false statements and omissions, defendant ... impeded the FBI's ongoing investigation into the existence of any links or coordination between individuals associated with the Campaign and the Russian government's efforts to interfere with the 2016 presidential election," Mueller's team wrote.

Court documents unsealed Monday in the Papadopoulos case outline how foreign nationals with ties to the Russian government were seeking to establish a relationship with at least one campaign official and that they offered "dirt" on Hillary Clinton.

Papadopoulos, 30, claimed in his first interview with the FBI that he made the contacts before Trump named him as a member of his campaign foreign policy team in March 2016, calling him an "excellent guy." He actually began communicating with them after joined the campaign as an energy expert.

He also downplayed the importance of the communications, telling the FBI that a professor living in London was "a nothing," while a Russian woman was just emailing him to say, "Hi, how are you?"

In fact, the professor had told Papadopoulos that Russians had "dirt" on Hillary Clinton. "They have thousands of emails," the professor said, according to the documents.

This was well before it was widely understood that Russia had hacked the Democrats, a fact first made public by a private cybersecurity firm in June.

Frank Figliuzzi, a former head of counterintelligence at the FBI, said on MSNBC’s Andrea Mitchell Reports that the Papadopoulos complaint portrays an effort by people likely connected to Russian intelligence to infiltrate the Trump campaign.

"He may not have actually understood who he was dealing with, but this has the fingerprints of Russian intelligence all over it," Figliuzzi said.

The professor introduced Papadopoulos to a Russian who said he was close to officials at the Russian Ministry of Foreign Affairs who then spoke with Papadopoulos over Skype about laying the groundwork for a meeting between the campaign and officials in Moscow.

The Russian woman — whom Papadopoulos mistakenly described in an email as the niece of Russian President Vladimir Putin — also tried to arrange a meeting between the Trump campaign and Russian government officials, the documents say.

Papadopoulos, 30, communicated with a "campaign supervisor" about his attempts to broker a meeting with the Russians to discuss U.S.-Russia ties during a Trump presidency, the court papers say.

"Great work," the supervisor, who was not named in the documents, told him in an email.

The court documents say that Papadopoulos' efforts were the subject of discussion in the campaign, with one official telling another in an email: "We need someone to communicate that DT is not doing these trips. It should be someone low level in the campaign so as not to send any signal."

After speaking with the FBI, Papadopoulos deactivated a Facebook account that had information about his communications with the foreign nationals, and he also got a new cellphone number, prosecutors said.

Papadopoulos, who spent 11 months on the Trump campaign, was arrested in July 2017 when he got off a plane in Washington, and he pleaded guilty on Oct. 5.

His plea agreement says the government will inform the sentencing judge of his "efforts to cooperate with the Government, on the condition that your client continues to respond and provide information regarding any and all matters as to which the Government deems relevant."

Papadopoulos' attorneys said in a statement that they would have no comment.

"We will have the opportunity to comment on George’s involvement when called upon by the Court at a later date," the statement said. "We look forward to presenting all the facts that led to the events that resulted in this charge."