WASHINGTON — Special counsel Robert Mueller’s investigation produced on Monday its first criminal charges, its first guilty plea and first public confirmation that an aide to President Trump’s campaign sought an allegiance with Russians to gather “dirt” on his political rival.

The series of charges unsealed early Monday morning is a significant step in the federal investigation of Russian attempts to influence the 2016 presidential election, which Trump has dismissed as a “hoax” and a “witch hunt.” While Mueller did not allege that Trump or his campaign colluded with Moscow, the charges offer the first public view into a far-flung criminal investigation that has cast a shadow on the president's first year in office.

In one case, ex-Trump foreign policy adviser George Papadopoulos pleaded guilty to a charge that he lied to FBI agents about his contacts with a professor he believed "had substantial connections to Russian government officials" during the campaign. The professor offered him "dirt" — in the form of thousands of emails — on Trump’s election opponent, Hillary Clinton.

A grand jury also indicted former campaign chairman Paul Manafort and his associate Rick Gates on charges that they secretly worked on behalf of pro-Russian factions in Ukraine, then laundered millions of dollars in profits through foreign bank accounts. Prosecutors charged that the men sought to cover up their work even while they held senior roles in Trump’s campaign. Both pleaded not guilty to the charges. A judge on Monday placed them under house arrest, setting bond at $10 million for Manafort and $5 million for Gates.

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At the White House, Trump and his aides sought to downplay the significance of the charges, suggesting that the work Manafort and Gates did in Ukraine was unconnected to the campaign and that Papadopoulos was a low-level official with little access or influence.

"Sorry, but this is years ago, before Paul Manafort was part of the Trump campaign," the president tweeted. "But why aren't Crooked Hillary & the Dems the focus?????"

Trump insisted that his campaign had no cooperation with Russia, which the U.S. intelligence community has accused of using cyberattacks and fake news to influence the election in favor of Trump. "Also, there is NO COLLUSION!" he tweeted.

White House spokeswoman Sarah Sanders said Mueller's charges have "nothing to do with the president ... nothing to do with the president’s campaign."

But Mueller did draw links between the charges and Trump's campaign. In a court filing signed by both prosecutors and Papadopoulos, Trump's former campaign aide acknowledged meeting with people he believed were connected to the Russian government while he was serving as an adviser to Trump. And the grand jury's indictment of Manafort and Gates alleged that they sought to cover up their Ukraine connections while they worked for Trump.

Sen. Mark Warner, the ranking Democrat and on Senate committee conducting its own investigation of Russian meddling, called Manafort’s indictment “a significant and sobering step in what will be a complex and likely lengthy investigation.”

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First guilty plea

Papadopoulos was arrested in July, after he returned to the United States from a trip to Germany. But a judge agreed to keep the case secret until Monday after Mueller's office said that he had agreed to cooperate with investigators. Revealing his arrest "could alert other subjects to the direction and status of the investigation," Mueller's team said in court documents.

The former aide acknowledged in a court filing that he misled FBI agents about his conversations with a professor who told him the Russian government had obtained "thousands of emails" with dirt on Clinton. The exchange came amid widespread publicity about Clinton's use of a private email server, and months before the anti-secrecy group WikiLeaks released internal Democratic National Committee emails that cast an embarrassing light on her campaign. U.S. intelligence agencies have concluded that the DNC's emails were hacked at the behest of the Russian government.

While collecting information on political opponents is routine in presidential campaigns, the involvement of a foreign government is remarkable. Papadopoulos' acknowledgement is the first link Mueller has publicly drawn between Trump's campaign and Russia.

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Papadopoulos admitted he met with a professor and a Russian woman in London in March 2016, after he joined the campaign. At the time, he mistakenly believed the woman was Russian President Vladimir Putin's niece.

After the London-based professor and a contact in the Russian Ministry of Foreign affairs expressed interest in a meeting with Trump, Papadopoulos in May 2016 sent an email to a high-ranking Trump campaign official with the subject line “Request from Russia to meet Mr. Trump,” according to the statement.

That email was forwarded to another campaign official with the message “Let[‘]s discuss. 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.” DT appears to be a reference to Trump.

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Papadopoulos was charged with misleading FBI agents about this and other exchanges when he met with them on Jan. 27. In court filings, he admitted that he made false statements, including by telling agents that he was told about "dirt" on Clinton before he began working for Trump. In fact, he acknowledged, it only occurred later, and that the professor who offered him the information seemed "uninterested" until he told him that he had joined the campaign.

In a written plea agreement, Mueller's office said it would agree to recommend that Papadopoulos face between zero and six months in prison. Papadopoulos' lawyers declined to comment on the case.

Former campaign chief indicted

Mueller's prosecutors also unsealed an array of charges against Manafort and Gates related to their work for the government of Ukraine, which began long before both men joined Trump's campaign. The charges include 12 counts of conspiracy, money laundering, failing to register as foreign agents and making false statements to investigators.

Manafort has long been a central figure in Mueller's investigation. FBI agents raided his apartment over the summer. He has come under scrutiny both for his work in Ukraine and his participation in a meeting in 2016 between Donald Trump, Jr. and a Russian lawyer promising damaging information about Clinton.

Gates had worked with Manafort in the private sector and followed him to Trump's campaign in 2016. Gates wound up moving to the Republican National Committee when Manafort was ousted from the campaign, and he helped set up a pro-Trump super PAC after the election.

After surrendering to FBI agents, Manafort and Gates appeared briefly in federal court in the afternoon. Both men surrendered their passports and are now required to remain confined in their homes, except for approved trips to consult with attorneys or receive medical treatment.

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Their indictment makes no reference to Manafort's work on Trump's campaign. But it alleges that Manafort's efforts to conceal his work on behalf of Ukraine continued while he was running the campaign. As late as Aug. 19, 2016, three days before Trump fired him, the indictment alleges that Manafort and Gates sent "false talking points" to one of the political consulting firms they had hired to lobby on behalf of pro-Russian factions in Ukraine.

Prosecutors allege that for more than a decade, Manafort and Gates worked secretly to influence the U.S. government on behalf of pro-Russian factions in Ukraine, then laundered their profits through a series of overseas businesses and bank accounts. In all, prosecutors alleged that $75 million passed through offshore bank accounts that the men controlled.

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Manafort "used his hidden overseas wealth to enjoy a lavish lifestyle in the United States," prosecutors wrote. Investigators traced wire transfers from bank accounts in Cyprus that Manafort allegedly used to pay his landscapers and to buy a Mercedes and three Range Rovers. Prosecutors also said that he used the accounts to pay more than $1.3 million to clothing stores in New York and Beverly Hills.

When U.S. authorities made inquiries about the payments last year, Manafort and Gates responded "with a series of false and misleading statements," prosecutors alleged.

Manafort's lawyer, Kevin Downing, called the charges "ridiculous" and said his work for Ukraine ended two years before he joined Trump's campaign. "President Donald Trump was correct," Downing said in a statement. "There is no evidence the Trump campaign colluded with the Russia government."

Gates spokesman Glenn Selig said he "welcomes the opportunity to confront these charges in court."

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Contributing: John Bacon and Steve Reilly in McLean, Va.