Special counsel Robert Mueller‘s office revealed in a court filing late Monday that Paul Manafort, President Donald Trump‘s former campaign chairman, routinely lied to federal investigators even after agreeing to fully cooperate.

Manafort’s actions violated a plea deal he reached in September. The terms of the agreement dictate that Manafort is not allowed to withdraw his guilty plea. According to The New York Times, Manafort’s defense attorneys said their client “believes he has provided truthful information.”

On Tuesday, The Guardian reported that Manafort secretly met with WikiLeaks founder Julian Assange several times. The final encounter between the two men allegedly took place “around March 2016,” just a few months before WikiLeaks published emails it had hacked from the Democratic National Committee’s servers during the summer of 2016. Earlier this month, charges against Assange were accidentally released in a secret court filing in Virginia.

In a tweet on Tuesday, WikiLeaks vehemently denied the allegations surrounding Assange and Manafort and slammed The Guardian, while calling the author of the report about the pair of men a “serial fabricator.”

In August, a jury in a highly publicized trial in Alexandria, Virginia, found Manafort guilty of eight counts of bank and tax fraud related to his work as a lobbyist for pro-Russia political candidates and organization in Ukraine, charges unrelated to his brief stint as Trump’s campaign chairman in 2016. A mistrial was declared for ten counts brought against Manafort, and it was later revealed that just one holdout juror prevented Manafort from being convicted on all 18 counts.

Manafort’s plea deal came on the same day Trump’s former longtime personal lawyer Michael Cohen pled guilty in New York to multiple campaign finance violations, including making hush money payments to women with whom the president had extramarital affairs.

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On Monday, Manafort’s lawyers reportedly requested that Judge Amy Berman Jackson set a sentencing date for their client, who has been kept in solitary confinement in a facility in Alexandria. Although the charges against Manafort aren’t directly related to his work for the Trump campaign in 2016, the most recent statement from Mueller’s office about the former campaign chairman shows how assertive the special counsel is being in probing which Trump campaign officials lied, what they lied about and how much they lied regarding their contacts with Russian officials during the election.

Another issue that many people have wondered is whether or not Trump will pardon Manafort for his crimes. After Manafort’s conviction in August, Trump called his former campaign chairman “brave” and said he felt “very badly” for Manafort because of the punishment he had received.

According to the Times, Monday’s court filing suggests “prosecutors do not consider Mr. Manafort a credible witness.”

Manafort is expected to serve at least ten years in prison for his ten charges, which include conspiracy to obstruct justice and bank and tax fraud.