WASHINGTON D.C. (WVEC/ABC NEWS) -- Former Trump campaign chairman Paul Manafort and his business associate, Rick Gates, have been indicted by a federal grand jury on charges of conspiracy against the United States and other felony charges.

George Papadopolous, a Trump campaign foreign policy adviser, pleaded guilty to making false statements to FBI agents.

Manafort and Gates are the first to be charged in the special counsel's five-month investigation into Russian meddling in the 2016 election and possible collusion between Trump campaign associates and Russia.

The indictments unsealed Monday in Washington contain 12 counts, including conspiracy to launder money, failing to register as a foreign agent, false statements, and multiple counts of failing to file reports for foreign bank accounts.

Manafort, of Alexandria, Virginia, and Gates, of Richmond, Virginia, both turned themselves in to the FBI on Monday.

Papadopolous pleaded guilty to making false statements and making material omissions during an interview with FBI agents in January. Papadopolous downplayed a meeting with a Russian professor with ties to Russian government officials and misled the agents about attempts to arrange a meeting between Russian government officials and the campaign.

Manafort originally emerged as a key figure in Special Counsel Robert Mueller’s inquiry because of consulting work he did in 2014 on behalf of the Ukrainian government.

In July -- the same month in which he registered as a foreign agent because of his lobbying work -- the FBI executed a search warrant at Manafort’s Virginia home, stemming from the Russia investigation. A source familiar with the matter described armed FBI agents’ waking Manafort early in the morning as they knocked on his bedroom door.

Manafort, 68, joined the Trump campaign in March 2016 as the campaign’s convention manager and was promoted to campaign chairman two months later. He stepped away from the campaign in August 2016 amid questions about his foreign business ties.

Gates, 45, joined Manafort’s international firm, Davis Manafort Partners, between 2006 and 2007. Gates’ connections to Trump before and after the election include his leading operations on the ground at the Republican National Convention in Cleveland, serving as a top deputy to inauguration chairman Tom Barrack on the 2017 Presidential Inauguration Committee.

Gates later joined the nonprofit America First Policies created after the election, a C4 committee which the president has endorsed.

Mueller was appointed special counsel in May by Deputy Attorney General Rod Rosenstein; Attorney General Jeff Sessions recused himself in March from all matters related to last year's presidential election.

In addition to the action taken by Mueller's team, Manafort has been heavily scrutinized by the multiple congressional committees conducting their own investigations into Russian meddling. In August, sources close to Manafort told ABC News that the former campaign chairman provided the committees with about 400 documents, including paperwork related to Ukraine.

Trump appeared ready for the other shoe to drop, taking to Twitter on Sunday to denounce the investigation.

President Trump’s attorney Ty Cobb later swatted down any connection.

“Contrary to what many have suggested, the president’s comments today are unrelated to the activities of the Special Counsel, with whom he continues to cooperate,” Cobb said in a statement to the White House press pool.

Paul Manafort and Richard Gates Indictment by 13News Now on Scribd

The Associated Press contributed to this report.