Manafort, who faces up to 10 years in prison for various financial crimes, had agreed to cooperate with investigators looking into possible collusion between President Donald Trump's campaign and Russian meddling in the 2016 election.
"Heroes will come of this, and it won't be Mueller and his bad Gang of Angry Democrats".
Because the plea agreement requires fulfillment of "each and every one" of the obligations that Manafort made and prohibits any criminal activity prior to sentencing, Mueller stated in the filing that Manafort has breached the agreement.
A source with direct knowledge of the matter told INSIDER at the time that Manafort's plea deal with Mueller was still intact.
"The Phony Witch Hunt continues, but Mueller and his gang of Angry Dems are only looking at one side, not the other", Trump tweeted. "Wait until it comes out how horribly & viciously they are treating people, ruining lives for them refusing to lie", Trump wrote on his Twitter account.
The brief but remarkable development indicates that not only has the former Trump campaign chairman shared extensive information with Justice Department prosecutors since he began cooperating, but that prosecutors also believe they are able to verify or refute that information.
Even though Mueller's team alleges he's breached his plea agreement, Manafort must comply with the prosecutors' demands going forward. Look at their past, and look where they come from. He was found guilty on eight counts, including tax fraud, money laundering and lying to the government.
"The now $30,000,000 Witch Hunt continues and they've got nothing but ruined lives", he wrote. Let these awful people go back to the Clinton Foundation and "Justice" Department! That means the judge overseeing his case will learn, in detail, of Manafort's alleged efforts to mislead investigators.
"The consequences are potentially devastating for Manafort", said Washington attorney Shanlon Wu, who represented Manafort's former associate Rick Gates before he pleaded guilty and became a star witness for prosecutors in Manafort's Virginia trial.
Both sides ask the judge to now move his case toward sentencing.
The judge in DC District Court who received the update Monday night has not yet weighed in, and a sentencing for Manafort's guilty plea has not yet been scheduled.
He faces a maximum sentence of 80 years in prison.
Trump also criticized the cost of the ongoing investigation.