The FBI believes a business associate of former Trump campaign chairman Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE had ties to Russian intelligence, including during the 2016 presidential race, prosecutors said in court documents filed Tuesday.

The documents, filed by special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's team, allege that Richard Gates, a longtime business partner of Manafort and President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE's former deputy campaign manager, was aware that an unidentified associate "was a former Russian Intelligence Officer."

The documents also allege that Gates and the individual continued to communicate with one another in the months before the 2016 presidential election. Such communication "was pertinent to the investigation," prosecutors said.

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The FBI's assessment was made public in a sentencing memo filed late Tuesday in the case of Alex van der Zwaan, a London-based lawyer, who pleaded guilty last month to lying to investigators about his contacts with Gates and the unidentified associate.

In the memo, prosecutors said that van der Zwaan acknowledged to special counsel investigators in an interview that Gates had told him of the associate's ties to a Russian military intelligence agency, the GRU.

The individual in question is referred to in the court filings only as "Person A." The Washington Post reported Tuesday, however, that the associate matches the description of Konstantin Kilimnik, a longtime Russian employee of Manafort's.

Kilimnik has previously denied having ties to Russian intelligence. He told the Post in a statement in June that he has "no relation to the Russian or any other intelligence service."

Mueller is charged with carrying out a criminal investigation into Russia's role in the 2016 presidential election and whether members of the Trump campaign conspired with Moscow to influence the race.

Manafort has found himself under intense scrutiny in that investigation. He has pleaded not guilty to a laundry list of charges, including from fraud, tax evasion, money laundering and failing to register as a foreign agent for his lobbying work abroad.

Gates, who was initially charged alongside Manafort in October, pleaded guilty last month to conspiracy and lying to FBI agents. He is cooperating with Mueller's investigation