The special counsel Robert Mueller is zeroing in on a series of conversations between Paul Manafort, the former Trump campaign chairman, and Konstantin Kilimnik, a former Russian intelligence operative, from August 2016 to March 2018.

Because much of the information in Mueller's court filing is redacted, it's unclear what the conversations were about.

But both Manafort and Kilimnik have previously acknowledged that they met in person on August 2, 2016. Manafort said he and Kilimnik discussed the Trump campaign and the recent hack of the Democratic National Committee. Kilimnik said they did not discuss the campaign but talked about "current news" and "unpaid bills."

Shortly after the August meeting, a private jet linked to Oleg Deripaska, a Russian-Ukrainian oligarch Manafort was indebted to, arrived in the US and was there for less than 24 hours.

The special counsel Robert Mueller's office submitted an extensive 31-page court filing late Tuesday that offers an unprecedented window into what Paul Manafort, the former chairman of President Donald Trump's campaign, told prosecutors after pleading guilty in the Russia investigation last year.

Much of Tuesday's court filing is redacted, as is an accompanying document containing approximately 70 exhibits.

But prosecutors did reveal some intriguing details about Manafort's dealings with investigators and the grand jury that could offer hints about where the Russia investigation is headed.

For one, they reaffirmed that Manafort testified to the grand jury about his communications with the former Russian intelligence operative Konstantin Kilimnik. According to the filing, prosecutors are interested in conversations Manafort and Kilimnik had about a certain topic from August 2, 2016, to March 2018. Much of the information about their interactions on this topic was redacted in the filing.

Several of their discussions were in person, prosecutors said.

Manafort has acknowledged that he met with Kilimnik in May and August 2016.

Three days before the August meeting with Manafort, Kilimnik wrote in an email to the Trump campaign chairman that he had "met today with the guy who gave you your biggest black caviar jar several years ago," referring to the Russian-Ukrainian oligarch Oleg Deripaska and his loans to Manafort.

"We spent about 5 hours talking about his story, and I have several important messages from him to you," Kilimnik wrote, adding, "I need about two hours because it is a long caviar story to tell."

Manafort is known to have offered Deripaska "private briefings" about the Trump campaign beginning in April 2016 and continuing until at least that July. Former intelligence officials told INSIDER the offer appeared to be part of an effort by Manafort to resolve a long-standing financial dispute with Deripaska.

Manafort said he and Kilimnik discussed the Trump campaign and the recent hack of the Democratic National Committee during the meeting on August 2, 2016. Kilimnik, meanwhile, said they did not discuss the campaign but talked about "current news" and "unpaid bills."

Shortly after the August 2 meeting, a jet linked to Deripaska landed in Newark, New Jersey, and was in the US for less than 24 hours.

Read more: Manafort's lawyers made a formatting error in a new court filing and accidentally revealed a slew of bombshells about his alleged lies to Mueller

Should Oleg Deripaska be worried? AP Photo/Sergey Ponomarev

Manafort is accused of misleading investigators about his meetings with Kilimnik and lying about sharing confidential Trump campaign polling data with him and discussing a Russia-Ukraine "peace plan" with him.

Mueller's work with the grand jury in the FBI's inquiry into Russia's election interference is typically shrouded in secrecy. But more details of Manafort's cooperation with prosecutors have slowly been made public since November, when Mueller first accused Manafort of lying to investigators in violation of his plea deal.

In December, the special counsel's office said in a court filing that Manafort told "discernible lies" about multiple topics, including his interactions with Kilimnik, a $125,000 payment to a firm related to a debt Manafort had incurred, his communications with Trump administration officials, and information relevant to another Justice Department investigation.

Earlier this month, Manafort's lawyers made a formatting error in a court filing and accidentally unsealed even more information about what he said, saying any lies were told unintentionally.

This week, Mueller revealed that Kilimnik is central to several threads in the Russia investigation, which is examining whether Trump's campaign colluded with Moscow to tilt the 2016 race in his favor and whether Trump sought to obstruct justice when he fired James Comey as FBI director in 2017.

The New York Times reported over the weekend that Comey's firing also prompted the bureau to launch a separate counterintelligence investigation into whether Trump was an agent of the Russian government.