Although the witnesses are unnamed, prosecutors have said they "were principals in a public relations firm that worked with Manafort in organizing a group of former European officials, known as the Hapsburg group, who promoted Ukrainian interests in Europe as well as the USA", according to The Associated Press.
Prosecutors said Mr Manafort has broken violations of his release while he awaits trial.
President Trump has spoken out about Manafort in recent days, slamming the Federal Bureau of Investigation and Department of Justice for not telling him that agents were "secretly investigating" Manafort during the 2016 presidential campaign.
The remaining include Michael Flynn, Trump's erstwhile national security advisor, who admitted guilt to one count of lying to investigators on December 1; Manafort's longtime partner and deputy campaign chair Rick Gates and campaign foreign policy advisor George Papadopoulos.
"Person D2 explained that he and Person D1 had been responsible for interfacing with the Hapsburg group, and acted as "intermediaries" between Manafort and Ukraine government officials and the Hapsburg group", prosecutors wrote.
Given the weight of the accusations, special counsel Robert Mueller asked a D.C. federal judge to consider revising the terms of his release-he is now under house arrest on a $10 million unsecured bail-or even send him to jail to await trial. Originally indicted in October, he has pleaded not guilty and is now confined to his home on a $10 million bond.
Despite the various indictments, Manafort has been out on bail and under house arrest, because he's being treated as a wealthy white man instead of a poor minority.
Manafort has also been indicted on federal tax-related charges in Alexandria, Virginia.
The motion says that after the indictment was made public, Manafort called Person D1 on a cell phone, but that the potential witness "sought to avoid Manafort" and "ended the call". The individual then hung up "because he was concerned about the outreach", according to the affidavit. He has pleaded not guilty. "I have made clear that they worked in Europe".