Paul Manafort's decision to plead guilty and flip in the Russia investigation is the single-biggest victory for the special counsel Robert Mueller, so far.

Before agreeing to a plea deal with Manafort, prosecutors likely sat down with him or his attorney for a proffer session, during which a defendant has to answer several key questions from investigators about his own case or any criminal activity he may have witnessed.

"This means Mueller's team feels that what Manafort has to offer is not just credible, but important," said one Justice Department veteran.

Manafort's decision to flip against President Trump likely blindsided his legal team.

Paul Manafort was the chairman of President Donald Trump's campaign when he offered a Russian oligarch "private briefings" on Trump's bid.

He was spearheading the campaign when WikiLeaks began dumping thousands of emails from the Democratic National Committee that had been stolen by Russian operatives.

Perhaps most importantly, he was one of three top Trump campaign officials to attend a meeting with two Russian lobbyists offering kompromat on Democratic nominee Hillary Clinton at the height of the campaign.

On Friday, Manafort pleaded guilty to two counts of conspiracy and obstruction, and Andrew Weissmann, a prosecutor working for the special counsel Robert Mueller, told a federal judge that Manafort had flipped and would be cooperating "in any and all matter as to which the government deems the cooperation relevant," including "testifying fully, completely" before a grand jury.

Patrick Cotter, a former federal prosecutor who worked with Weissmann in the past, didn't mince words when he reacted to the development.

"Manafort's cooperation is the single most important advancement for the Mueller probe," he said. "He is the single most important witness thus far, because his position was such that he can shed light on the most critical question of what the president knew, and when he knew it."

'This is a huge get for Mueller's team'

Robert Mueller. Alex Wong/Getty Images

Mueller is investigating Russia's interference in the 2016 election, whether members of the Trump campaign colluded with Moscow to tilt the race in his favor, and whether Trump sought to obstruct justice after the existence of the investigation became public knowledge last year.

News of Manafort's deal with Mueller came after intense speculation over whether the former Trump campaign chairman would plead guilty or go to trial.

Trump's lawyer, Rudy Giuliani, told Politico earlier this week that Manafort was in a joint defense agreement with the president, and that Trump's team was not worried about Manafort flipping.

He told BuzzFeed early Friday, less than two hours before Manafort's plea hearing, that Manafort had not withdrawn from the agreement, in what appeared to be an indication that even if Manafort entered a guilty plea, he would not be cooperating against Trump.

For that reason, Weissmann's announcement that Manafort had flipped likely "blindsided" the president's legal team, Cotter said.

But Elie Honig, a former Justice Department lawyer who prosecuted hundreds of organized crime cases, said it's common in high-profile cases for a cooperator to stay silent about their agreement with the prosecution until the last minute.

"Joint defense agreements are very common in mob cases," he said. "And when you're working to flip someone in a joint defense agreement, they'd have to keep it a secret. The last people you'd tell are the other people in the agreement because they could threaten you, try to dissuade you, or do other things to derail you."

Giuliani told Business Insider on Friday evening that Manafort had still not withdrawn from his joint defense agreement with Trump.

But Jeffrey Cramer, a longtime former federal prosecutor who spent 12 years at the Justice Department, pointed out that a joint defense agreement in this case is not the same as those in most other federal cases.

"Normal defense agreements involve people on the same indictment," he said, adding that he believed it was likely the most Manafort was getting out of his agreement with Trump was money to cover his legal expenses. "Regardless, he is talking to Mueller now, so I doubt there will be many meetings between the two sides going forward."

What Manafort knows is important for several threads of the Russia investigation, like the hack of the DNC and any communication between Trump campaign members and Russian interests. But the biggest value he brings to Mueller is the ability to shed light on the controversial June 2016 meeting between campaign officials and Russian lobbyists.

Manafort attended the meeting along with Donald Trump Jr. and Jared Kushner, and it eventually emerged that, contrary to Trump Jr.'s initial statements, the meeting was pitched as "part of Russia and its government's support" for Trump's candidacy.

It is a federal crime to accept something of value from a foreign government in connection to an American election, and legal experts have suggested that if Trump campaign officials took the meeting to get kompromat on Clinton, it could place them in serious legal jeopardy.

"Manafort has knowledge of that meeting because he was there, there's no speculation on that," Cramer said. "He knows what it was for, what happened at the meeting, and he may even know about the cover-up afterward. This is a huge get for Mueller's team."

'It's getting very lonely on Trump Island'

Donald Trump. Alex Edelman/Getty Images

After Manafort's plea deal was announced, both Giuliani and the White House released statements downplaying its significance.

"Once again an investigation has concluded with a plea having nothing to do with President Trump or the Trump campaign," Giuliani said in a statement to Business Insider. "The reason: the President did nothing wrong."

White House press secretary Sarah Sanders struck a similar chord.

"This had absolutely nothing to do with the President or his victorious 2016 Presidential campaign," she said in a statement. "It is totally unrelated."

While the charges to which Manafort pleaded are unrelated to Trump, legal experts say Manafort will tell Mueller a lot more than just information about what was in the indictments against him.

"Sanders' statement is just false," Cramer said. "Some things are gray. This is really black and white. The Russia meeting was obviously during the campaign, the DNC hack was obviously during the campaign, as were many other events Manafort may know about."

"The court of public opinion is one thing; the legal system doesn't care about the spin the White House is putting on this," he added.

Honig agreed.

Manafort's cooperation, he said, is a massive victory for the special counsel, because "the way it works with federal cooperation is it's all or nothing."

"The cooperator doesn't just talk about select people or categories, with the exception of close family members at times," he said. "They have to talk about everything they've ever done, all the criminal activity they knew about, every crime they've committed."

Before striking a plea deal with a defendant, prosecutors sit down with them or their attorney for what's known as a proffer session, which involves answering any questions from investigators, including those about their own case and other possible criminal activity they may have witnessed.

Even if Mueller's team didn't have the chance to sit down for a full proffer session with Manafort, experts said he almost certainly knew the broad strokes of what the former Trump campaign chairman had to offer.

Ultimately, prosecutors only agree to a cooperation deal with a defendant if the defendant gives them information that can be confirmed by other witnesses and information investigators have gathered.

Cotter, who was part of the team that convicted the Gambino crime family boss John Gotti after flipping his right-hand man, Salvatore "Sammy the bull" Gravano, said that was the most significant takeaway for him.

"This means Mueller's team feels that what Manafort has to offer is not just credible, but important," he said. "That suggests that what Manafort knows is really critical evidence about that Trump Tower meeting, who knew about it and when, and what other contacts were there between the campaign and people around Trump."

The bottom line? Every day, Cotter said, there are one or two fewer people the president can rely on.

"It's getting very lonely on Trump Island."