WASHINGTON — The FBI has found that a business associate of former Trump campaign chairman Paul Manafort had ongoing ties to Russian intelligence, including during the 2016 campaign when Manafort and his deputy, Rick Gates, were in touch with the associate, according to new court filings.

The documents, filed late Tuesday by prosecutors for special counsel Robert Mueller, also allege that Gates had said he knew the associate was a former officer with the Russian military intelligence service.

The allegations underscore Mueller's interest in Manafort and Gates, who continued to interact with business associates in Ukraine even as they helped lead Donald Trump's presidential campaign.

Manafort, 68, has pleaded not guilty to conspiracy, money laundering, tax fraud and bank fraud charges related to his lobbying work for a Russian-friendly political party in Ukraine and former Ukrainian President Viktor Yanukovych.

Gates, 45, who was deputy campaign manager for Trump and had earlier worked with Manafort in Ukraine, pleaded guilty last month to conspiracy and lying to the FBI in a cooperation deal with Mueller's inquiry.

Prosecutors made the allegation without naming the Manafort associate but described his role with Manafort in detail. The description matches the Russian manager of Manafort's lobbying office in Kiev, Konstantin Kilimnik.

Kilimnik, a White House spokesman and an attorney for Gates did not immediately respond to requests for comment.

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

A spokesman for Manafort, who is under a court gag order, declined to comment.

The information about the FBI's assessment of the Manafort associate came in a court filing related to the upcoming sentencing of London lawyer Alex van der Zwaan, whose firm worked with Manafort when he served a political consultant in Ukraine.

Van der Zwaan, 33, the son-in-law of a prominent Russian-Ukrainian banker, pleaded guilty last month to lying about his September 2016 contacts with Gates and the Manafort associate, identified in court documents only as "Person A."

Prosecutors explained that van der Zwaan had lied and withheld documents about information that was "pertinent" to their investigation — that Gates had been in direct contact during the presidential campaign with a person who "has ties to a Russian intelligence services and had such ties in 2016."

They said when van der Zwaan was interviewed by the FBI in November, he told investigators that Gates had informed him that Person A was a former GRU officer.