Special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE has assembled a list of figures cooperating with his Russia investigation that could provide him with substantial insight into the workings of the Trump campaign.

Mueller’s ability to turn associates of President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE into cooperators has been a key facet of his investigation, lending strength to a probe that has pressed on for nearly a year and a half amid withering public scrutiny.

Legal analysts expect former Trump campaign chairman Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE and other recruits to bring the special counsel closer to getting to the bottom of whether there was collusion between the Trump campaign and Moscow, though doing so may hinge on Mueller striking deals with even more figures.

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With Mueller’s probe advancing behind closed doors, it is impossible for onlookers to judge the value or extent of any one witness’s cooperation.

At the same time, observers say the deals Mueller has struck signal he believes their cooperation to have significant value.

“If they have struck a deal where they’re going to cooperate, then that’s a pretty good indication that special counsel’s office believes they have something worth cooperating over,” said Jack Sharman, a former special counsel to Congress for the Whitewater investigation.

In Manafort, the newest cooperator, the special counsel has a window into the June 2016 Trump Tower meeting between top campaign aides and a lawyer with connections to the Russian government.

The central question surrounding the Trump Tower meeting and other significant events is whether members of the campaign conspired with Russia to damage Democrat Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE’s presidential ambitions, and to what level any such conspiracy rose in the campaign.

Onetime national security adviser Michael Flynn is also viewed as valuable to Mueller’s investigation because of his role in the campaign and, briefly, the administration, as well as his own contacts with Russians.

Flynn pleaded guilty last December to lying to FBI investigators about his discussions with then-Russian Ambassador Sergey Kislyak about sanctions on Moscow during the presidential transition. Court documents show he also talked about those conversations with other members of the Trump campaign.

Mueller asked a federal court to move forward with Flynn’s sentencing last month, a signal the special counsel believes he has gleaned all the information he can from the former adviser, and a sentencing date has been set for Dec. 18.

Other campaign associates also have been ensnared in the investigation.

Richard Gates, Manafort’s longtime business partner, pleaded guilty to conspiracy and false statements charges in February and began to cooperate with Mueller’s team in their case against Manafort related to illegal foreign lobbying.

It was Gates’s testimony that ultimately helped prosecutors secure a guilty verdict against Manafort on eight counts of bank and tax fraud in Virginia federal court over the summer. Manafort was due to begin a second trial in D.C. in September, but instead agreed to plead guilty and work with prosecutors.

Others have agreed to assist the special counsel but are not considered key figures.

Richard Pinedo, a California man who pleaded guilty and cooperated in the case against the Internet Research Agency, a Russian troll farm, was sentenced to six months in prison. Mueller also secured an obscure cooperator in Sam Patten, a GOP operative and former Manafort associate. Neither was involved in the Trump campaign.

The first person known to be cooperating with Mueller’s probe was George PapadopoulosGeorge Demetrios PapadopoulosTale of two FBI cases: Clinton got warned, Trump got investigated Trump says he would consider pardons for those implicated in Mueller investigation New FBI document confirms the Trump campaign was investigated without justification MORE.

Mueller’s team, however, has signaled that it gleaned little from the former campaign adviser, whose guilty plea made headlines in October 2017 when it revealed he had been told the Russians possessed “dirt” on Clinton in the form of “thousands of emails” — before WikiLeaks began releasing hacked emails from the Democratic National Committee.

“It was at best begrudging efforts to cooperate and we don't think they were substantial or significant in any regard,” Andrew Goldstein, one of Mueller’s prosecutors, said at Papadopoulos’s sentencing hearing last month.

Papadopoulos was sentenced to 14 days in federal prison for lying to FBI agents about his Russia contacts. He subsequently told CNN he has “no recollection” of sharing information about emails possessed by the Russians but couldn’t “guarantee” it.

Mueller’s team has also reportedly interviewed Michael Cohen, Trump’s former personal attorney, though no formal agreement cementing his cooperation has been publicly released.

Sharman, the former special counsel, said the value of various people now talking to Mueller’s team could vary depending on what offenses they are looking into and who they are considering charging with crimes.

He also acknowledged the opacity of the matter, something exacerbated by the fact that Mueller never speaks to the press.

“Nobody knows outside of that office and outside of the grand jury, what the substance of anyone’s cooperation is,” Sharman said.

Trump has long scorned the Mueller investigation as a "witch hunt" and claimed charges against Manafort and others have nothing to do with his presidential run.

He acknowledged in August, however, that the Trump Tower meeting was predicated on getting “information on an opponent,” undermining a statement by his eldest son, Donald Trump Jr. Don John Trump'Tiger King' star Joe Exotic requests pardon from Trump: 'Be my hero please' Zaid Jilani discusses Trump's move to cancel racial sensitivity training at federal agencies Trump International Hotel in Vancouver closes permanently MORE, last year that it primarily focused on American adoptions of Russian children. The participants have said that the meeting did not ultimately bear fruit.

“My son’s a good young guy. He did what every other person in Congress would do if somebody came up to them, said, ‘Hey, I have information on your opponent,’” Trump told The Associated Press in an interview Tuesday. “There was nothing wrong with having an opposition research meeting and nothing happened from the meeting.”

Manafort, who attended the meeting along with Trump Jr. and Jared Kushner Jared Corey KushnerAbraham Accords: New hope for peace in Middle East Tenants in Kushner building file lawsuit alleging dangerous living conditions Trump hosts Israel, UAE, Bahrain for historic signing MORE, Trump's son-in-law and senior adviser, would have been privy to discussions concerning the meeting and could potentially speak to Trump’s knowledge of it. The president claims he had no advanced knowledge of the meeting.

“He can talk about the conversations that took place before, during and after,” said Seth Waxman, a former federal prosecutor in D.C.

“I would put Manafort leaps and bounds above everyone else simply because of the time and effort the government put in to flip him,” Waxman said. “When the government goes as hard and as deep on someone like Manafort, it’s because they want him for a purpose and they believe he has very valuable information.”

Manafort’s value extends beyond his involvement in the Trump Tower affair. Having spent five months as campaign chairman, Manafort could answer questions about the softening of language in the Republican Party’s platform on Ukraine and any possible accords with the Russians.

Former federal prosecutors also expect Mueller’s team to question Manafort on whether the campaign had advanced knowledge of Democratic emails hacked by Russia.

Court filings and proceedings have offered glimpses of the information provided by other players.

It was not initially clear whether Gates, who also worked on the Trump campaign and later on the transition, was cooperating beyond the Manafort case. However, a recent filing from his attorney suggests he is helping Mueller on other aspects of the investigation.

In a motion asking the court to remove Gates’s GPS tracker and lift some of his travel restrictions last week, his attorney, Tom Green, wrote that Gates’ interviews with the special counsel’s team “have been numerous and they continue to this day.”

Neither Manafort nor Gates have been sentenced, though Manafort will appear in federal court in Virginia on Friday as Judge T.S. Ellis III looks to move forward with his sentencing for the bank and tax fraud charges.

There is broad agreement that Mueller is unlikely to take major overt steps in the investigation before the midterm elections, reflecting what has become routine practice of Justice Department officials to avoid moves that could be construed as political close to an election.

Bloomberg, citing anonymous officials, reported Wednesday that Mueller is expected to issue findings on his inquiries into collusion and obstruction of justice after the elections, under pressure from Deputy Attorney General Rod Rosenstein Rod RosensteinDOJ kept investigators from completing probe of Trump ties to Russia: report Five takeaways from final Senate Intel Russia report FBI officials hid copies of Russia probe documents fearing Trump interference: book MORE to complete the probe as quickly as possible.

Trump said Tuesday that his legal team was reviewing a series of written questions that Mueller had submitted for him on the collusion inquiry.

“We are looking at certain questions having to do with the word collusion,” Trump told the AP. “Of course there was no collusion. So we are looking at that, and we’ll make a determination.”

Meanwhile, recent grand jury appearances by associates of Roger Stone Roger Jason StoneThe agony of justice Our Constitution is under attack by Attorney General William Barr Justice IG investigating Stone sentencing: report MORE suggest that Mueller is circling the longtime Trump ally, who has been scrutinized for his links to WikiLeaks.

It is possible that Mueller has brought other cooperators into the fold who are unknown to the public; this could include individuals who have plea agreements that are under seal, who have not yet been charged, or who have voluntarily agreed to cooperate.

“Your job as a prosecutor is to go as high up the chain of the organization as you can and prosecute the most culpable people and put an end of their criminal conduct,” said Joyce Vance, a former U.S. attorney in Alabama.

“He’ll want to keep going so that the people who he prosecutes are the people who are the most responsible for any criminal conduct he uncovers. No prosecutor wants to stop at the midway point, [though] sometimes you have to because you don’t acquire enough evidence to go higher,” Vance said.