One of special counsel Robert Mueller's top prosecutors said this week an August 2016 meeting between former Trump campaign chairman Paul Manafort and Konstantin Kilimnik, his old associate who the FBI claims has ties to Russian intelligence, is at the "heart" of the federal Russia investigation.

Andrew Weissmann, who the New York Times has dubbed Mueller's "legal pit bull," made the revealing statement to a federal judge in a closed hearing Monday, according to a highly redacted D.C. court transcript released Thursday.

He said the meeting, which reportedly took place at the Grand Havana Room in New York City and was also attended by former Manafort associate Rick Gates, is at “the heart of what the Special Counsel's Office is investigating."

Weissmann further told U.S. District Judge Amy Berman Jackson, "This goes to the larger view of what we think is going on, and what we think the motive here is."

It's unclear what transpired during that meeting, but the court filing, which also reveals Kilimnik was in Washington, D.C., for President Trump's inauguration, offers a rare look into what Mueller's team is focusing on in what is widely expected to be the end of the investigation into potential collusion between the Trump campaign and the Kremlin.

Earlier this year, Manafort's attorneys accidentally revealed through botched redactions that their client shared polling data before the election with Kilimnik. The court filing also revealed Manafort had been accused by Mueller of lying about discussing a Ukrainian peace plan with Kilimnik during the 2016 campaign.

Kilimnik, a Russian national, was indicted by Mueller last June on charges of obstruction of justice and tampering with a witness on behalf of Manafort.

Mueller's team believes Manafort breached the terms of his plea deal by making false statements about his interactions with Kilimnik, as well as contacts with members of the Trump administration and a $125,000 loan he received in 2016, according to ABC News. Manafort denies that he intentionally misled investigators.

On Monday, Berman Jackson delayed Manafort's sentencing date in March by a week.

Mueller’s team accused Manafort, who pleaded guilty in 2018 to conspiracy against the U.S., of lying to investigators in November. In September, he pleaded guilty in a second trial in Washington, D.C., to two counts of conspiracy.

Manafort was first convicted in August on eight counts of bank and tax fraud by a jury in Virginia. Sentencing for that case is scheduled for Feb. 8 in Alexandria, Va.

Manafort faces the possibility of spending the rest of his life in prison.