Paul Manafort deliberately lied to investigators and a federal grand jury in the investigation conducted in Russia, a judge ruled.
The decision of US District Judge Amy Berman Jackson was another loss for former Trump campaign president, who faces jail terms in two separate criminal cases stemming from the investigation of Robert's special lawyer Mueller.
This compromises Manafort's chances of obtaining a reduced sentence, although the judge said she would decide the exact impact when sentencing next month.
This four-page order resolves a dispute that had improved understanding of how Mueller viewed Manafort's actions in his wider investigation into Russian interference in the elections and any possible coordination with US President Donald Trump.
The judge concluded that there was sufficient evidence to assert that Manafort had broken his plea agreement by lying about three of the five cases that prosecutors had mentioned.
This included his misrepresentations about the FBI, prosecutors and a federal grand jury about his interactions with Konstantin Kilimnik, his co-defendant who, according to the FBI, had connections with Russian intelligence.
Prosecutors had accused Manafort of lying about his talks with Kilimnik about a possible peace plan for Ukraine.
At a sealed hearing, Mueller's attorney, Andrew Weissmann, said that one of the discussions – a meeting on Aug. 2, 2016 at the Grand Havana Club cigar bar in New York – resulted in a "new" meeting. wider view of what we think is going on "and what" we think the motive here is ".
"This is, I think, at the heart of the investigations of the Office of the Special Council," said Mr Weissmann, adding that "this meeting and its events are of particular importance to the special council".
The meeting took place while Manafort was still playing a leading role in the Trump campaign. Rick Gates, a longtime MP from Manafort and also Trump's campaign assistant, was also present.
Prosecutors also accused Manafort of lying about sharing survey data with Kilimnik during the 2016 presidential campaign.
Manafort's lawyers said he did not deliberately mislead the investigators, but instead left out details until his memory was refreshed.
They also stated that the special advocate had not demonstrated that the subject matter was important to the investigation.
In his decision, the judge stated that the prosecutors had not sufficiently supported two allegations against Manafort, claiming that she could not conclude that he had deliberately lied about Kilimnik's role in falsifying witnesses or what Manafort had told investigators about his contacts with the Trump administration.