Here are the top takeaways:
President Donald Trump has been saying -- and tweeting -- a similar refrain for two years now: "NO COLLUSION!"
Trump says Barr's summary proves him right. "(T)he investigation did not establish that members of the Trump Campaign conspired or coordinated with the Russian government in its election interference activities," the letter states.
Trump and his allies are already seizing on that line of the summary released Sunday, and it's undoubtedly a major headline pushing back on two years of Democrats claiming they had evidence of collusion.
Of course, numerous Trump world contacts with Russians during the campaign and transition have already been established. So what does it mean that Mueller didn't find conspiracy or coordination?
Barr's summary includes a footnote that explains how Mueller defined coordination: an "agreement -- tacit or express -- between the Trump Campaign and the Russian government on election interference."
In addition, Barr wrote that no Trump associate conspired or coordinated "despite multiple offers from Russian-affiliated individuals to assist the Trump campaign."
No exoneration for obstruction
The Justice Department decided not to prosecute the President for obstructing justice with his behavior -- both public and in private -- but Trump isn't fully cleared. He is likely to still face a hailstorm of political criticism for his actions after taking office, when more of Mueller's investigation is revealed.
Mueller said he thoroughly investigated the obstruction question, though he didn't interview Trump. Ultimately he left the question to the President's political appointees, Barr and Deputy Attorney General Rod Rosenstein, to decide what to do.
Barr and Rosenstein chose not to prosecute. They decided that they couldn't press a case against Trump because they didn't have evidence of an underlying crime, Barr's letter said Sunday.
Barr also said the President's actions did not "constitute obstructive conduct" and were not "done with corrupt intent." (His and Rosenstein's decision hadn't factored in the constitutional briar patch of indicting a sitting president.)
"While this report does not conclude that the President committed a crime, it also does not exonerate him," Mueller wrote in his report, as Barr quoted him on Sunday.
Perhaps without collusion, there could be no obstruction, Shan Wu, a CNN contributor who previously represented Rick Gates in the investigation, said on Sunday. It's possible Barr didn't want to indict the President because an obstruction charge wouldn't stand without an underlying collusion case.
Notably, Barr has always been opposed to an obstruction case against the President. Before he was attorney general under Trump, Barr wrote a memo in June 2018 to top Justice Department officials, saying he believed an obstruction case against Trump was "fatally misconceived."
Trump was never interviewed
Trump's other big win during the Mueller probe was that he never sat for an interview.
His personal legal team had long been opposed to what they believed could be a perjury trap and Mueller never put the President in that position, in either a private interview or in testimony under oath.
Barr's letter on Sunday suggests Mueller never needed that interview.
Mueller appears not to have gotten there because the Russian collusion case hadn't materialized. But he did consider forcing an interview with the President. CNN reported Sunday that Mueller had deliberated at length about subpoenaing Trump before deciding not to make a formal request within the Justice Department.
Barr previously told Congress that Mueller never split with his bosses about steps to take in the investigation.
Ultimately, Mueller was left with a set of written answers from Trump about questions of collusion and contact with Russians. These were heavily vetted by Trump's lawyers, and because of the President's legal team's negotiations and lack of pressure from Mueller, didn't touch on obstruction of justice.
The political fight is just beginning
Congressional Democrats are ready to pick up the torch of the Mueller investigation.
Lawmakers are already raising questions about Barr making the prosecution decision on obstruction of justice, and House Judiciary Chairman Jerry Nadler says he will call on the attorney general to testify "in the near future."
"In light of the very concerning discrepancies and final decision making at the Justice Department following the Special Counsel report, where Mueller did not exonerate the President, we will be calling Attorney General Barr in to testify before @HouseJudiciary in the near future," the New York Democrat tweeted Sunday.
Republicans, meanwhile, are joining Trump in claiming vindication for the President — and they're going on offense to criticize Democrats over collusion.
"For years, high-ranking Democrats in leadership positions in Congress claimed the special counsel's report would provide definitive proof of collusion, and today's report has proven that to be an outright lie once and for all," said House Minority Whip Steve Scalise of Louisiana.
It's a political fight of competing narratives that will surely spill into the 2020 presidential election.
Barr's summary ramps up calls for full report
The four-page summary Barr released on Sunday is written in a lawyerly way in many cases, especially when it comes to the decisions made over the probe into obstruction of justice.
And the letter -- in addition to Barr's previously expressed opposition to an obstruction probe into Trump in the first place -- is fueling Democratic demands that Mueller's full report be released.
"Given Mr. Barr's public record of bias against the Special Counsel's inquiry, he is not a neutral observer and is not in a position to make objective determinations about the report," House Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer said in a joint statement Sunday.
Of course, Republicans have also said they want Mueller's report to be released and several have said the report should be out so that Democrats cannot claim the Justice Department is hiding anything.
There's a lot more information to come
Documents from Mueller. Internal Justice Department discussions. Witness interview memos. Thousands of court records.
There are still many pages of documentation of Mueller's work -- plus discussions of his and the attorney general's decisions and findings -- that aren't yet public. But those documents could be available in the future, even soon, and would fill out even more pieces of the puzzle about what Mueller learned.
Parts of Mueller's original report could still become public, Barr said on Sunday, and the attorney general promised Congress he'd work "expeditiously" and is working with Mueller "as quickly as possible" on a public release.
Barr wrote that Mueller will be involved in scrubbing the report to remove secret grand jury information and any content related to ongoing investigations before it could be made public. (The ongoing investigations are "several" and other prosecutorial offices will be considering "further action," Barr wrote.)
Separately, Barr alludes to internal discussions that may have be documented, thus making it possible those papers could someday see the light of day. Barr cited instances during the obstruction of justice review when he and Rosenstein consulted with the Office of Legal Counsel. Mueller also considered subpoenaing Trump for an interview over several months, CNN reports, and Barr says in his letter Mueller spoke with other department officials in weighing obstruction case decisions, potentially creating even more of a paper trail.
Separately, the release of documentation of court action will be largely out of Barr's hands -- and that ream of filings is massive.
Federal judges will likely be asked to decide how and when to release the mostly still-confidential 500 search warrants, almost 300 warrants for electronic data and 13 requests to foreign government Barr said Mueller used to gather evidence. So far, only a few dozen warrants related to the cases of Paul Manafort and Michael Cohen are available publicly.
Mueller also interviewed about 500 witnesses, Barr said, meaning there are that many more files from those discussions kept by the Justice Department and FBI.
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