Washington (CNN) Donald Trump's 2016 campaign chairman Paul Manafort continued to keep in touch with his Russian associate Konstantin Kilimnik and discuss Ukrainian politics while he was working for the campaign and through last year, and that's become a central issue of Robert Mueller's special counsel investigation, according to a transcript of a closed-door court hearing.

A special counsel prosecutor also suggested in the hearing that the former campaign chairman was not forthcoming because Manafort was angling for a pardon.

Mueller prosecutor Andrew Weissmann told a federal judge Monday that a meeting Manafort and Kilimnik had in August 2016 was "of significance to the special counsel." A redacted version of the transcript was released Thursday.

At the meeting, Ukrainian policy -- and, it appears according to previous filings, a peace plan that would have benefited Russia -- came up. Manafort in interviews with the special counsel said the discussion with Kilimnik about the topic ended then.

But, prosecutors say, Manafort met with Kilimnik several times throughout 2017 and even into 2018, and discussed Ukrainian policy again and again.

"This goes, I think, very much to the heart of what the special counsel's office is investigating," Weissmann said at the hearing. "There is an in-person meeting at an unusual time for somebody who is the campaign chairman to be spending time, and to be doing it in person."

Weissmann also noted that at the August 2016 meeting of Kilimnik, Manafort and his deputy Rick Gates took pains to leave separately — at a time when contacts between the Trump campaign and Russia were of utmost interest.

Prosecutors have previously outlined how Manafort's Ukrainian connections mattered to their probe, in that the Ukrainians Manafort had long worked for had pro-Russian leanings and ties to Russian President Vladimir Putin.

Manafort even went back to work in Ukrainian politics in 2017, and conducted a poll for a Ukrainian politician that included questions about the topic he discussed with Kilimnik. The 2018 polling work took place after he was indicted.

"The Office of Special Counsel contends that Mr. Manafort lied about the number of times they discussed it, that he and Mr. Kilimnik had not just discussed it once on August 2nd, 2016, but also in December of 2016; in January 2017, in person, in Washington, D.C., when Kilimnik was here for the inauguration; in February of 2017, including in person on [REDACTED]; and even in the winter of 2018," Weissmann said, according to the transcript.

The August 2016 meeting with Kilimnik also mattered to prosecutors because the FBI has linked Kilimnik to the Russian intelligence service, the GRU, Weissmann said.

At one point in the discussion, Weissmann describes how Manafort told prosecutors that his topic of discussion with Kilimnik was a "backdoor [REDACTED] ... and because of that, he was not going to countenance it." Weissmann didn't explain the significance of the reference to a "backdoor."

The meeting is of such interest still to the prosecutors that Manafort testified about it before a federal grand jury late last year. No apparent criminal cases have been made public yet related to the topic.

At one point, the prosecutor acknowledges that Manafort may have been playing for a pardon. He acknowledges that while Manafort was lying, he had multiple motivations -- even while Gates had given the Special Counsel's Office other information. He did not want "negative consequences in terms of the other motive that Mr. Manafort could have, which is to at least augment his chances for a pardon," Weissmann said.

Trump declined in late November to rule out a potential pardon.