WASHINGTON – The world got its first glimpse on Sunday into the investigation that has cast a cloud over Donald Trump's presidency.

After two days reading through Special Counsel Robert Mueller's confidential report, Attorney General William Barr sent a letter to Congress offering a summary of the investigation into Russian interference in the 2016 presidential election and possible links to Trump's campaign.

In Barr's letter, he said that no proof was found that the Trump campaign conspired with Russia but said no decision was made over whether the president obstructed justice. Barr said that Mueller's office left that charging decision to him and the Justice Department.

"The Special Counsel states that 'while this report does not conclude that the President committed a crime, it also does not exonerate him,'" the letter states, quoting from the special counsel investigation.

Barr added that after review, he and Deputy Attorney General Rob Rosenstein "have concluded that the evidence developed during the Special Counsel's investigation is not sufficient to establish that the President committed an obstruction-of-justice offense."

Trump reacted to the news on Twitter as he prepared to leave Mar-a-Lago, his private resort in Palm Beach, Fla., writing "No Collusion, No Obstruction, Complete and Total EXONERATION. KEEP AMERICA GREAT!"

But Barr's letter and his summary of the Mueller probe is really just the beginning, with lawmakers already pushing for the complete release of the investigation and underlying evidence.

Here's what we know, what we don't know, and what we might never find out:

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Did Mueller find collusion?

Nope, according to the attorney general.

"The Special Counsel's investigation did not find that the Trump campaign or anyone associated with it conspired or coordinated with Russia in its efforts to influence the 2016 U.S. presidential election," Barr wrote in a letter to Congress summarizing Mueller's investigation.

Barr noted that this came "despite multiple offers from Russian-affiliated individuals to assist the Trump campaign."

This was seen as a win for the president, who for years has railed against the investigation and repeatedly said there was no collusion with Russia.

"It was just announced that there was no collusion with Russia, the most ridiculous thing I've ever heard. There was no collusion, folk," Trump said as he boarded Air Force One on the way back to Washington.

Did the president obstruct justice?

That isn't really clear.

After years investigating, Mueller didn't say yes or no. Instead, he offered the evidence he had to Barr.

Mueller "did not grat a conclusion — one way or the other — as to whether the examined conduct constituted obstruction," Barr notes in his letter to Congress. He noted Mueller's office laid out evidence "on both sides" of the question as to whether Trump committed a crime.

"The Special Counsel states that 'while this report does not conclude that the President committed a crime, it also does not exonerate him,'" Barr wrote, quoting from Mueller's investigation.

Barr said after examining the investigation, departmental policies and talking with Deputy Attorney General Rod Rosenstein, he decided there was not enough proof that Trump committed a crime.

"I have concluded that the evidence developed during the Special Counsel's investigation is not sufficient to establish that the President committed an obstruction-of-justice offense," Barr wrote.

What happens Next?

This letter tees up a massive push to have the entirety of the report released to the public.

Democrats have been very vocal in pushing for everything, including the underlying evidence, to be released. After Barr's letter, House Judiciary Committee Chairman Jerry Nadler, D-N.Y., said he would be calling Barr to testify before the committee in coming days.

Barr's letter was just a written summary of Mueller's findings – essentially parts of the report that Barr believes he can share in accordance with Justice Department rules. The only information Barr is required to reveal is whether Mueller's bosses overruled his investigative actions. And Barr said that did not happen.

How big is the report?

We don't know. The Justice Department said it was "comprehensive," but didn't elaborate. A security officer from Mueller's office delivered a printed copy to the offices of Deputy Attorney General Rod Rosenstein, who turned it over to Barr on Friday afternoon.

Will we get to read the Mueller report?

It's too soon to tell.

What's certain is that there will be a fight in Congress to make the report public. Lawmakers and Trump have indicated that they want the report to become public. But it's unclear if that will stand after the nation found out Mueller could not determine whether the president committed a crime.

House Speaker Nancy Pelosi said in a conference call with Democrats on Saturday that she would reject any classified briefings of congressional intelligence leaders, known as the "Gang of Eight." Any briefings, she said, should be unclassified so the American public can be informed, according to an official who was on the call.

Barr told lawmakers that he was "committed to as much transparency as possible." But he did not say he would release Mueller's complete report. Instead, he said he would consult with Mueller and Rosenstein to "determine what other information from the report can be released consistent with the law ... and the Department's long-standing practices and policies."

Barr has said that the department often cannot reveal information about grand juries, or "derogatory" information about people who have not been charged with a crime.

What happens to Mueller?

Peter Carr, Mueller's spokesman, said the special counsel would be "concluding his service in the coming days." A small staff would remain to close down Mueller's office. The office had already confirmed that some of his prosecutors and investigators were leaving for other jobs.

So Mueller came up empty?

The investigation has led to charges against some of Trump's closest advisers.

And we know what investigators believe the 13 Russian nationals and a Russian troll farm did to disrupt U.S. politics and tilt the presidential election in Trump's favor.

It's still unclear whether Mueller's report will speak to a pattern among those cases. "That's part of what the Mueller report is for. One of the things the Mueller report will do is it will attempt to put all of the indictments Mueller has brought into an overall context," said Patrick Cotter, a white-collar defense attorney and a former federal prosecutor.

What did we know already?

In hundreds of pages of court filings, Mueller's office revealed an extensive Russian intelligence operation that used hacking, stolen documents and phony social media campaigns to sow discord in U.S. politics and support Trump's campaign for the presidency. It revealed that some of Trump's aides worked eagerly to benefit from that operation, seeking damaging information from Russians even as Trump was seeking out business in the country. And it revealed that many of Trump's aides then lied to Congress, federal investigators and the public to downplay those connections.

What about other people?

Trump's eldest son, Donald Trump Jr., and his son-in-law, Jared Kushner, for example. They, along with Manafort, met on June 9, 2016 at Trump Tower with Russian attorney Natalia Veselnitskaya and several other Russians. The meeting occurred after Trump Jr. was promised it would yield dirt on Democratic presidential candidate Hillary Clinton. That meeting was a focal point of Mueller's investigation, but the fact that no one has faced charges for it suggests Mueller's team didn't think it amounted to a crime.

All the lies?

The special counsel's investigation revealed an extensive pattern of perjury. Several of Trump's former advisers, including Flynn, Cohen and Stone, either admitted to or were accused of lying. Will Mueller address that in his report? Cotter hopes so.

"Because to a federal prosecutor, I think that it is a strikingly unusual aspect on this whole case. That so many people lied to investigators," Cotter said.

Is this the end?

No.

Mueller referred his investigation on Cohen's criminal activities to federal prosecutors in the Southern District of New York, who opened an inquiry into illegal hush-money payments to silence two women who claimed to have had sex with Trump. Justice Department investigators have also been examining lobbying firms tied to Manafort, and Trump's inaugural committee.

The department hasn't said whether Mueller's office made any other referrals.

Contributing: Kevin Johnson, Christal Hayes, Eliza Collins and Bart Jansen

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