Washington (CNN) Former Trump campaign chairman Paul Manafort spoke to a federal grand jury last fall about his communications with Konstantin Kilimnik, including regarding an in-person meeting with the Russian associate and an email with him, special counsel Robert Mueller revealed Tuesday.

The 31-page filing gives an unprecedented window into Mueller's work with the grand jury, which is typically secret. It's also the first confirmation from prosecutors that Kilimnik is still central to the grand jury's efforts.

Based on recent filings from Mueller's team, Kilimnik appears to be at the heart of pieces of Mueller's investigation into Russian interference in the 2016 presidential election.

The special counsel kept more details about the meeting secret in its Tuesday filing, such as what the in-person meeting was about and when it took place. An accompanying document with about 70 exhibits is also mostly redacted

Mueller's team has been redacting information in its recent court filings to protect other individuals and to keep secret its ongoing investigations, of which there are multiple still in the works. But the special counsel has not redacted Kilimnik's name.

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