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The FBI says that during the 2016 campaign Trump campaign chair Paul Manafort and his deputy, Rick Gates, were in touch with a Manafort associate who had an ongoing relationship with Russian intelligence, according to court documents filed late Tuesday by prosecutors for Special Counsel Robert Mueller.

The filings also allege that Gates stated he was aware the Manafort associate had once been an officer with Russian military intelligence (GRU). The Manafort associate is not named, but is referred to as "Person A" and described as someone who the FBI says "has ties to a Russian intelligence service and had such ties in 2016."

Gates pleaded guilty in February to conspiracy and making false statements and is cooperating with Mueller's investigation of possible collusion between the Trump campaign and Russia.

Manafort has pleaded not guilty to charges that include bank fraud and money laundering in connection with his work for a pro-Russia political party in Ukraine.

Gates had also worked with Manafort in Ukraine.

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The Mueller team provided the FBI's evaluation of the Manafort associate in a pre-sentencing report for another defendant who has pleaded guilty in the probe, London-based attorney Alex van der Zwaan. Prosecutors said van der Zwaan lied about his contact with Gates, and with Gates's contact with Person A during the 2016 presidential campaign.

While working for the London office of the Skadden Arps law firm, Van der Zwaan had helped compile a 2012 report commissioned by Manafort to defend the prosecution of former Ukrainian Prime Minister Yulia Tymoshenko. Manafort worked as a consultant for the government of Ukrainian President Victor Yanukovych, which had imprisoned Tymoshenko.

During his first interview with the special counsel's office in 2017, according to the report, "van der Zwaan admitted that he knew" Person A was connected to Russian intelligence, "stating that Gates told him Person A was a former Russian intelligence officer with the GRU."

But prosecutors say van der Zwaan, who is married to the daughter of a Russian billionaire, "lied repeatedly" to Mueller's prosecutors and "destroyed and failed to produce pertinent documents."

According to prosecutors, van der Zwaan, Gates and Person A had a series of conversations in September and October 2016, including a phone call that van der Zwaan recorded. Prosecutors say Van der Zwaan concealed the recorded phone call from them.

"The conversations concerned potential criminal charges in Ukraine about the Tymoshenko report and how the firm was compensated for its work," according to the pre-sentencing report.

"In van der Zwaan's recorded conversation with Person A, in Russian, Person A suggested that 'there were additional payments,' that '[t]he official contract was only a part of the iceberg,' and that the story may become a blow for 'you and me' personally."

Attorneys for Richard Gates and Alex van der Zwaan did not immediately respond to requests for comment.

A spokesperson for Paul Manafort declined to comment. Manafort remains under a court-imposed gag order.