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Former Trump campaign deputy chairman Rick Gates communicated with a person with ties to a Russian intelligence service in late 2016, Special Counsel Robert Mueller stated in a new court filing.

The revelation came during the sentencing of Alex Van Der Zwaan, an attorney connected to Gates and former Trump campaign chairman Paul Manafort, who pleaded guilty last month to making false statements to the FBI.

“That Gates and Person A were directly communicating in September and October 2016 was pertinent to the investigation,” the Tuesday court filing, obtained and reviewed by Fox News, read.

Mueller is investigating Russian meddling and the actions of Trump campaign associates during the presidential election.

The unnamed person, cited as “Person A,” had lived in Kiev and Moscow up until August 2016 and worked with Manafort and Gates in connection with their Ukraine lobbying work. Person A, according to the filing, is a foreign national and was a “close business colleague” of both Manafort and Gates, working for Manafort’s company in Ukraine, Davis Manafort International, LLC.

The communications were disclosed in what's known as the “Government’s Sentencing Memorandum."

“The lies and withholding of documents were material to the Special Counsel’s Office’s investigation. That Gates and Person A were directly communicating in September and October 2016 was pertinent to the investigation,” the filing read. “Federal Bureau of Investigation Special Agents assisting the Special Counsel’s office assess that Person A has ties to a Russian intelligence service and had such ties in 2016.”

The filing said that van der Zwaan admitted, during his first interview with the special counsel’s office, that he know of the connection between Gates and “Person A,” stating that “Gates told him Person A was a former Russian Intelligence Officer with the GRU,”—meaning Gates was aware of the unnamed person’s ties during his communications.

Just last month, Mueller motioned to dismiss a charges brought against Gates in a superseding indictment in Virginia federal court, following Gates’ guilty plea to the first round of charges brought forth in October 2017.

A federal grand jury returned the new charges in the superseding indictment brought against both Gates and Manafort, which included conspiring against the United States, conspiring to launder money, failing to register as an agent of a foreign principal and providing false statements. The superseding indictment was separate from the original charges brought against both Manafort and Gates in October.

Manafort continues to plead not guilty, and just Tuesday, his attorneys motioned to dismiss the superseding indictment brought against him in the same court. Manafort’s lawyers asserted that Mueller exceeded the scope of their investigation, and claimed Mueller wasn’t even investigating their client for any potential collusion.

Manafort’s attorney Kevin Downing filed the motion in federal court Tuesday to dismiss the indictment, noting that “the charges against Mr. Manafort do not ‘arise directly from’ the special counsel investigation.

“Indeed, the special counsel has never suggested that Mr. Manafort had anything to do with alleged ‘coordination [with] the Russian government,’ or even that he is investigating Mr. Manafort on that subject,” Downing wrote in the court filing.

Despite Gates’ guilty plea, Manafort has said that he will “continue to maintain my innocence,” and that he “hoped and expected” Gates would have had “the strength to continue the battle to prove our innocence.”

Earlier this month, U.S. District Judge T.S. Ellis III, who is assigned to the superseding indictment in Virginia, suggested that Manafort could face life in prison, and “poses a substantial flight risk” because of his “financial means and international connections to flee and remain at large.”

“Specifically given the nature of the charges against the defendant and the apparent weight of the evidence against him, defendant faces the very real possibility of spending the rest of his life in prison,” Ellis wrote.

Fox News’ Bill Mears contributed to this report.