A secret meeting that Paul Manafort had in August 2016 with a former business partner with ties to Russian intelligence “goes … to the heart” of the special counsel’s investigation, prosecutors told a federal judge in a closed-door hearing earlier this week.

“This goes, I think, very much to the heart of what the special counsel’s office is investigating,” Andrew Weissmann, a prosecutor working for special counsel Robert Mueller, said at a hearing on Monday.

Weissmann was referring to an Aug. 2, 2016 meeting between Manafort and Konstantin Kilimnik at the Havana Club in New York City. Details of what Manafort and Kilimnik allegedly discussed are blacked out on a transcript unsealed on Thursday.

But the cryptic reference by Weissmann suggests that prosecutors believe that issues related to the 2016 campaign were discussed at the meeting. Mueller’s main task when appointed special counsel in May 2017 was to investigate whether Trump associates conspired with Russian agents to influence the 2016 election.

No Trump associates have been charged with conspiring with Russian agents. Several have been charged with lying to the FBI about various Russia-related matters, however. Manafort, a longtime GOP consultant, has been convicted in the special counsel’s probe for crimes related to his pre-campaign consulting work for the Ukrainian government.

At the time of the meeting with Kilimnik, Manafort served as chairman of Trump’s campaign. He was fired less than two weeks after the release of news reports about his work on behalf of former Ukrainian President Viktor Yanukovych.

Manafort and Kilimnik were known to have worked together on behalf of Yanukovych through 2014. They also appeared to have discussed working for a Ukrainian politician through last year, well after Manafort was indicted in the special counsel’s probe.

Weissmann said in the hearing that the timing of Manafort’s meeting raised red flags for prosecutors because it occurred “at an unusual time for somebody who is the campaign chairman to be spending time, and to be doing it in person.”

Weissmann also said that the FBI has assessed Kilimnik as having “a relationship with Russian intelligence.”

“That meeting and what happened at that meeting is of significance to the special counsel,” Weissmann added.

Kilimnik also served as a go-between for Manafort and Oleg Deripaska, a Russian aluminum magnate with ties to Vladimir Putin. (RELATED: Manafort Offered To Provide Briefings To Oleg Deripaska During Campaign)

Emails that Manafort sent during his stint on the campaign show that he told Kilimnik that he would be willing to offer Deripaksa briefings about the Trump campaign.

“If he needs private briefings we can accommodate,” Manafort wrote Kilimnik on July 7, 2016.

Manafort also suggested he would use his unpaid role on the Trump campaign to settle a business dispute that he had with Deripaska at the time.

“How do we use to get whole?” Manafort wrote to Kilimnik in April 2016, just after joining the Trump campaign.

Deripaska sued Manafort in 2014, alleging that Manafort had stolen $19 million from a Ukrainian business venture.

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