Prosecutors say Manafort acknowledged the meeting only after being told that they were in the same city on the same day.
The information was accidentally revealed in a defense filing Tuesday and was meant to be redacted.
While some of the redacted information was not a big deal, there was one section of the failed redaction that appears to shed light on Mueller's understanding of Manafort's roll in the 2016 presidential election.
The filing also showed that Mueller accused Manafort of discussing a "Ukraine peace plan" with Kilimnik and meeting with him in Madrid at some point during the 2016 campaign. His attorneys added that "it is not surprising at all that Mr. Manafort was unable to recall specific details prior to having his recollection refreshed".
Manafort misled prosecutors in debriefings about his communications and a meeting with Konstantin Kilimnik, an associate with ties to Russian intelligence, according to a December 7 filing by Mueller. (See Doc. 460 at 6). He also asked Kilimnik to offer private briefings about the campaign to Oleg Deripaska, a Russian businessman who is close to Putin and to whom Manafort owed money.
Mueller could potentially bring new charges against Manafort. During a proffer meeting with prosecutors on September 11, Manafort told investigators "he would have given the Ukrainian peace plan more thought" had he not been working on Trump's campaign at the time.
During the 2016 Republican national convention, the Trump campaign came under scrutiny for changing a policy proposal for Ukraine in a way that made it favorable for Russian Federation. Manafort spokesman Jason Maloni said Tuesday the Madrid trip mentioned in the filing occurred in January or February 2017 - months after Manafort was ousted from the campaign and as Trump was taking office.
Manafort's attorneys insisted in the filing that the exchange did not constitute "outreach" to the president.
He has been wheelchair bound for months, his lawyers say, because of gout, and has been kept essentially in solitary confinement for his own safety.
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Secretary of State Mike Pompeo, with the two agreeing that no politics be injected into the extradition process. Canadian consular officials saw them once each in mid-December.
Both men were indicted in special counsel Robert Mueller's investigation into Russian election interference and possible coordination with the Trump campaign.
Manafort provided "complete and truthful information to the best of his ability", the lawyers wrote in the 10-page filing.
"These circumstances weighed heavily on Mr Manafort's state of mind and on his memory as he was questioned at length", the lawyers wrote.Those alleged lies were said to involve issues including Manafort's contacts with Trump administration officials.
In another faultily redacted portion, Manafort's attorneys revealed that an unnamed third party texted Manafort last May asking to use his name as a point of introduction if the person met with Trump.
The judge in DC federal court who has overseen his case, Amy Berman Jackson, gave him the opportunity to respond before she would hold a hearing about the facts of the situation.
Manafort was found guilty in August on eight counts of tax and bank-fraud in a Virginia case related in part to his work as an unregistered foreign lobbyist.
The ex-Trump aide had been hoping for leniency at his 5 March sentencing under the plea deal.
Although Manafort's attorneys could request an evidentiary hearing on the claim he lied to Mr. Mueller's team, they suggested the matter be addressed in a pre-sentencing report compiled by the probation office.