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’s investigation into Russian interference in the election has yielded numerous bombshell developments, while enduring consistent attacks from 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 and his allies.

Thirty-seven have been charged in connection with the probe; eight have pleaded guilty; and six, including four Trump associates, have agreed to cooperate.

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But more significant than what has been learned through Mueller’s public filings, perhaps, are the remaining unknowns. Mueller has not answered the central question of whether there was collusion between Trump’s campaign and Moscow, nor has he made a judgment on whether the president obstructed justice.

Here is where the investigation stands 16 months after it began.

THE COOPERATORS

Several individuals Mueller has brought charges against have agreed to plead guilty and cooperate, turning Trump associates into potential witnesses.

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 was the first to plead guilty to lying to FBI agents about his Russia contacts and cooperate last October. Papadopoulos, who Trump claims played a minimal role on the campaign, tried to leverage his contacts to broker a meeting between the campaign and officials in Moscow.

Mueller’s prosecutors have signaled his cooperation failed to generate significant leads, and he was sentenced to 14 days in jail earlier in September.

Likely more significant, observers say, are the cooperators Mueller has in former national security adviser Michael Flynn and 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.

Flynn pleaded guilty in December to lying to FBI agents about his discussions with the Russian ambassador to the U.S. about sanctions. A key campaign surrogate who went on to serve briefly in Trump’s Cabinet, Flynn would have insight into the dealings of the campaign and the early days of the administration.

Mueller moved forward with Flynn’s sentencing this month, a signal that his team is finished debriefing the former Trump adviser. Still, Mueller can summon Flynn to testify in any trials. Flynn will be sentenced in December, after the midterm elections.

Earlier this month, Manafort pleaded guilty to two conspiracy charges in a deal that allowed him to avert a second federal trial in Washington, where he faced charges related to his foreign lobbying for pro-Russian forces in Ukraine. Trump’s lawyers have dismissed Manafort’s case as unrelated to the campaign.

His testimony is viewed as potentially valuable given his participation in the June 2016 Trump Tower meeting arranged between 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, 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, and a Russian lawyer said to possess damaging information on 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.

Manafort could also be important to other foreign lobbying investigations spun off from Mueller’s probe; the special counsel has reportedly passed a collection of these cases off to federal prosecutors in New York.

Richard Gates, a longtime business partner of Manafort, pleaded guilty and began cooperating in February after originally being charged alongside Manafort. Gates testified against him in a dramatic court appearance in August, after which Manafort was found guilty on eight counts of bank and tax fraud.

Gates worked on Trump’s campaign and on the presidential transition team, though it remains unclear to what extent he may be cooperating beyond the Manafort inquiry. He has yet to be sentenced.

Mueller also secured a lesser-known cooperating witness in Sam Patten, a Republican political operative and Manafort associate who pleaded guilty to a foreign lobbying violation in late August. Mueller passed his case off to the U.S. attorney’s office in D.C., but his plea agreement includes cooperation with the special counsel.

Patten does not have any apparent ties to Trump, but court filings in his case revealed a stunning detail: Patten allegedly concealed foreign payments for tickets to Trump’s inauguration that were sought by a Ukrainian oligarch.

Richard Pinedo, a California man who pleaded guilty to identity fraud in connection with Mueller’s case against a Russian troll farm, has also cooperated and will be sentenced in October.

Others key players have also voluntarily interviewed with the special counsel, including Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsRoy Moore sues Alabama over COVID-19 restrictions GOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs MORE, a former Trump campaign surrogate, and Don McGahn, the White House counsel. The New York Times reported in August that McGahn shared details about the president’s actions during James Comey James Brien ComeySteele Dossier sub-source was subject of FBI counterintelligence probe Judge will not dismiss McCabe's case against DOJ Democrats fear Russia interference could spoil bid to retake Senate MORE’s firing as FBI director and other events relevant to the obstruction inquiry.

THE RUSSIANS INDICTED

Mueller’s probe has ensnared more than two dozen Russians accused of crimes linked to a multifaceted plot to influence the 2016 election.

In February, the special counsel indicted 13 Russian nationals and three Russian organizations linked to the Internet Research Agency, a shadowy troll farm operation in St. Petersburg that used social media to spread divisive content to American audiences before the election.

In July, Mueller charged 12 Russian intelligence officers in the hacking of the Democratic National Committee and U.S. electoral systems. The Russians are alleged to work for the GRU, an intelligence service with Russia’s military known for its brazen operations.

The Russians are unlikely to see their day in court, though two U.S.-based lawyers have surfaced to fight charges against Concord Management and Consulting, a Russian firm accused of funding the troll farm that is allegedly controlled by Yevgeny Prigozhin, an associate of Russian President Vladimir Putin nicknamed “Putin’s chef.”

Mueller also charged Russian national Konstantin Kilimnik alongside Manafort in June with witness tampering. According to court filings, Kilimnik worked for an international offshoot of Manafort’s consulting business, running an office in Kiev.

Kilimnik is also said to be the unnamed Russian referenced by prosecutors in filings related to Patten’s case who worked with him to broker meetings between a Ukrainian oligarch and U.S. officials and to arrange the inauguration payments.

THE OTHER GUILTY PLEAS

Dutch lawyer Alex van der Zwaan, the son-in-law of a Russian billionaire, pleaded guilty in February to lying to FBI agents in connection with the inquiry into Manafort and Gates’ foreign lobbying and was sentenced to 30 days in prison in April.

His case does not appear to have any direct links to the collusion probe, though a court filing made by the government shed new light on contacts between campaign officials and Moscow. Prosecutors wrote in a March filing that Gates was communicating with an unnamed individual, “Person A,” in September and October 2016 who Mueller believes “has ties to a Russian intelligence service and had such ties in 2016.” “Person A” is widely suspected to be Kilimnik.

Michael Cohen Michael Dean CohenJudge orders Eric Trump to comply with New York AG's subpoena before Election Day A huge deal for campaign disclosure: Trump's tax records for Biden's medical records Our Constitution is under attack by Attorney General William Barr MORE, Trump’s former personal attorney, has also pleaded guilty to federal fraud charges in connection with Mueller’s investigation. Similar to the Patten case, Mueller referred information on Cohen to the U.S. attorney’s office in the Southern District in New York, resulting in federal authorities raiding his office and hotel room back in April.

During the heat of the Manafort trial in August, Cohen pleaded guilty to eight charges and in court testimony implicated the president in a scheme that involved paying off women to keep information about their alleged affairs with Trump from surfacing during the 2016 campaign.

While his plea deal in New York does not include cooperation, Cohen, through his lawyer Lanny Davis, has been sending signals that he is willing to cooperate with Mueller.

ABC News reported last week that Cohen has interviewed with Mueller’s team and was questioned about Trump’s past dealings with Russia, to include financial and business matters, as well as potential collusion. Davis, through a spokesman, declined to comment. Davis is an opinion contributor for The Hill.

THE UNKNOWNS

The developments in Mueller’s probe have in some ways raised more questions than they answer, resembling pieces of a complex puzzle that has not been put together.

The special counsel’s recent moves have signaled he is zeroing in on former Trump adviser 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, though it remains unclear whether more individuals will be charged in the investigation, or whether there will be more trials.

It is equally unclear how Mueller will wrap up his investigation, or when. Many expect the special counsel to write a report to the Justice Department laying out his findings, but it is also unknown whether that report would be made public.

Mueller also has yet to secure an interview with the president, and there have been no reports of the special counsel interviewing Trump Jr., a key player in the Trump Tower meeting.

The most consequential outstanding question, those closely watching the investigation say, is what Mueller’s inquiry will reveal about Trump, his family and their involvement in any collusion.

“There are a bunch of puzzle pieces left,” said Steven Cash, a lawyer at Day Pitney who specializes in criminal and national security law. “The most important are, what did the president and his family know and were they complicit in it?”

It could be several weeks before the public sees the next major development in the probe. There is widespread speculation that Mueller will adhere to typical Justice Department practice and avoid taking steps that could be viewed as inappropriate too close to the midterm elections.

“I don’t see Mueller making any big moves before the election,” said Ron Hosko, a former FBI assistant director who worked under Mueller at the bureau.