A breakdown in the cooperation agreement between former Trump campaign chief Paul Manafort and special counsel Robert Mueller suggests prosecutors believe Manafort told them substantial lies, law enforcement experts said.
On Monday, attorneys for Manafort and the special counsel called off a cooperation deal that had been in place since Manafort pleaded guilty to a range of financial crimes in September. The special counsel office said in a court filing that Manafort had lied "on a variety of subject matters" after the deal was struck, and it promised to provide details to the court before Manafort's sentencing.
"It has to be a pretty big deal," said Stephanie Douglas, a 24-year FBI veteran and a former senior official in the bureau's national security branch. "It's not that he lied about the color of a jacket that he wore to a meeting."
In the same joint filing, Manafort's lawyers wrote that the former lobbyist and political operative "believes he has provided truthful information and does not agree with the government's characterization or that he has breached the agreement."
Manafort is one of several Trump associates Mueller has targeted in his probe into possible links between the Trump campaign and the Russian government. The latest allegation could signal a setback for Mueller. Trump's public praise of his erstwhile campaign chief has raised the specter of a possible presidential pardon — a possibility that his lawyers have done little to downplay.
Trump has repeatedly denied any collusion with Russia took place and has labeled the year-and-a-half-long investigation a "witch hunt."
David Weinstein, a defense attorney and former federal prosecutor in the U.S. Attorney's Office in Miami, said that misstating minor facts would be less likely to cause a "significant hangup" for investigators than providing false information that is substantive to the probe.
"That's going to be a bigger problem" for Manafort, Weinstein said. "Not that it's ever the right thing to do to lie, but some lies are more material than others."
The White House did not immediately respond to CNBC's request for comment. A spokesman for the special counsel declined to comment.
The collapse of the plea deal means that Manafort could soon be sentenced to prison without the reduction that was expected to be provided in return for his cooperation. For Mueller, the breakdown is also a setback. Experts say that Manafort is unlikely to prove useful as a witness now that the special counsel has said he is a liar.
"It's a lose-lose for everybody," said Nick Gravante, a partner of New York-based law firm Boies Schiller. "For Mueller, any evidence from Manafort that he was planning to give is rendered essentially useless."
Still, while any information Manafort provided will likely be unusable for the purposes of witness testimony, if he provided any accurate information, it could still be used to generate leads, Douglas said.
"He can't be used as a witness at a trial or in a criminal matter, but I think he can still provide information on the counterintelligence side," she said. That information could be useful if he discussed his interactions with intelligence or business officials overseas, she said.
Before joining Trump's campaign in March 2016, Manafort operated a global consulting business and worked for pro-Russia politicians in Ukraine, among others.
Mueller may also be able to use any information Manafort provided about reported dealings with Julian Assange. The Guardian reported that Manafort held secret talks with the WikiLeaks founder in the Ecuadorean embassy in London on multiple occasions between 2013 and 2016. WikiLeaks denied the report.
Mueller has accused Russian intelligence officials of using WikiLeaks to disseminate information during the 2016 campaign to hurt Trump's rival Hillary Clinton. CNBC has not been able to independently confirm the Guardian report.
Manafort's potential sentence could become more severe as a result of Monday's court filing. Before the legal development, Manafort faced up to 10 years in prison on charges brought in Virginia as well as up to five years for the charges outlined in his cooperation deal. If he is prosecuted and convicted for the new allegations, the sentence could grow, virtually ensuring a life sentence for the 69-year-old.
Mueller's prosecutors have been aggressive in pursuing cases against those who they believe have misled them. The special counsel has obtained guilty pleas from four individuals for lying to federal investigators since Mueller was appointed in May 2017.
"Manafort is either an incredibly stupid criminal or he's protecting some secret so big that he's willing to spend the rest of his life in jail to keep the world from discovering it," Matthew Miller, a former Justice Department spokesperson and a partner at the strategic communications firm Vianovo, wrote in a post on Twitter.
It is also possible that Manafort is angling for a presidential pardon, some experts said.
The president's attorney, Rudy Giuliani, suggested to NBC News on Tuesday that it was "conceivable" that "the special counsel in their zeal to get the president may be going too far."
Trump, in a Tuesday morning tweet storm, vented rage at Mueller for "ruining lives" of people involved in the probe who are "refusing to lie."
"Mueller is a conflicted prosecutor gone rogue," Trump added.
If Trump offers Manafort a pardon, Mueller has little legal recourse. The president has vast authority to issue pardons unilaterally for any reason. In an interview with the New York Daily News published Tuesday, Giuliani said that there had been no recent discussions about a potential pardon for Manafort. But he didn't rule out the possibility.
"Pardons are never really ruled out," he said.