The communications were an "effort to influence their testimony and to otherwise hide evidence", FBI Special Agent Brock Domin wrote in a declaration filed to court alongside Mr Mueller's motion.
A motion submitted by Special Counsel Robert Mueller, who is heading up the main investigation into alleged Russian election meddling, said that Manafort had contacted witnesses by phone and encrypted text messaging for the goal of securing "materially false testimony".
The motion alleges that Mr Manafort sent messages to two members of "The Hapsburg Group", a lobbying firm he worked with to promote the interests of Ukraine.More news: Plastic peril: Whale dies after swallowing 80 shopping bags
That person subsequently told the investigators that he, as well as Manafort himself, was well aware of the fact that the group had extended its lobbying to the USA and said that he saw Manafort's intervention as an attempt to influence his testimony.
"As only one of two people left who could become President, why wouldn't the Federal Bureau of Investigation or Department of "Justice" have told me that they were secretly investigating Paul Manafort (on charges that were 10 years old and had been previously dropped) during my campaign", the president wrote on Twitter.
Manafort has denied any wrongdoing and pleaded not guilty.
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".More news: Long wait over for Goldolphin as Masar wins Derby
One minute after sending the news article, Manafort wrote: "We should talk".
However, the charges don't relate to campaign activities, they range from money laundering and failing to register as a foreign agent to bank and tax fraud. Mueller's team claims Manafort has been engaged in a witness tampering, and therefore violated the conditions of his release.
The person told investigators that he interpreted Manafort's efforts to reach him as a way to influence his potential statements. The witness said he hung up because he was "concerned" about the call, according to the affidavit. "I have made clear that they worked in Europe". Around that same time, Manafort's co-defendant and longtime business associate, Rick Gates, pleaded guilty and agreed to cooperate with prosecutors.
Court papers also accuse Person A of making several attempts to influence the witnesses' testimony in February and later in April. Both witnesses said that wasn't true.More news: Telegram app now GDPR-compliant as Apple approves update amid Russian Federation friction