The letter describes "two main" Russian efforts to influence the election including “attempts by a Russian organization… to conduct disinformation and social media operations in the United States” and “the Russian government’s efforts to conduct computer hacking operations” targeting former Secretary of State Hillary Clinton’s presidential campaign and the Democratic Party.
In both circumstances, the “Special Counsel did not find that the Trump campaign, or anyone associated with it, conspired or coordinated with the Russian government in these efforts.”
The special counsel’s office made no conclusion on the matter of possible obstruction of justice by President Donald Trump.
"The Special Counsel, therefore, did not draw a conclusion – one way or the other – as to whether the examined conduct constituted obstruction," the letter read. "Instead, for each of the relevant actions investigated, the report sets out 'difficult issues' of law and fact concerning whether the President's actions and intent could be viewed as obstruction."
In his communique to lawmakers, Barr underscored that the special counsel states that “while this report does not conclude that the president committed a crime, it also does not exonerate him.”
While Mueller’s report did not reach a conclusion as to whether obstruction of justice occurred, Attorney General Barr’s letter said he determined a case for obstruction was not warranted.
“In cataloging the President’s actions many of which took place in public view, the report identities no actions that, in our judgment, constitute obstructive conduct, had a nexus to a pending or contemplated proceeding, and were done with corrupt intent, each of which, under the Department's principles of federal prosecution guiding charging decisions, would need to be proven beyond a reasonable doubt to establish an obstruction-of-justice offense."
Barr said his intent is to release as much as possible but there are grand jury secrecy concerns with some portions.
The White House celebrated the news, with President Donald Trump hailing the report on Twitter and, later in comments to reporters, as an "exoneration."
"This was an illegal takedown that failed and hopefully, someone is going to be looking at the other side," Trump told reporters on Sunday.
He tweeted: "No Collusion, No Obstruction, Complete and Total EXONERATION. KEEP AMERICA GREAT!"
"The Special Counsel did not find any collusion and did not find any obstruction," White House press secretary Sarah Sanders said in a statement. "Attorney General Barr and Deputy Attorney General Rosenstein further determined there was no obstruction. The findings of the Department of Justice are a total and complete exoneration of the President of the United States.”
Jay Sekulow, one of President Trump’s lawyers, told ABC News chief anchor George Stephanopoulos that the report was a "complete win for the president and the American people."
"Not just a win." Sekulow said. "The entire basis upon which this inquiry was instituted was a concern over collusion between the Russian government and the trump campaign, Bob Mueller and the Department of Justice could not be clearer. To have the obstruction issue even as a viable one, which it was not, there would have to be an underlying crime, which there wasn't."
House Judiciary Chairman Jerry Nadler, D-N.Y., questioned the timing of the letter tweeting "Special Counsel Mueller worked for 22 months to determine the extent to which President Trump obstructed justice. Attorney General Barr took 2 days to tell the American people that while the President is not exonerated, there will be no action by DOJ." He later tweeted that "in light of the very concerning discrepancies and final decision making at the Justice Department following the Special Counsel report," the House Judiciary Committee will be calling Barr in to testify in the near future.
In his letter to members of Congress on Sunday, Barr described the breadth of the special counsel’s extensive and exhaustive probe. Mueller’s 19 prosecutors and 40 FBI agents issued more than 2,800 subpoenas, executed nearly 500 search warrants, and interviewed approximately 500 witnesses, Barr wrote, the results of which amounted to 37 indictments and six guilty pleas to date.
The news comes amid Democrats continued calls for the full release of the findings.
Senate Minority Leader Chuck Schumer and House Speaker Nancy Pelosi, in a joint letter, seemed to sum up the sentiment of many congressional Democrats in saying that Barr's letter "raises as many questions as it answers" and that Congress "requires the full report and the underlying documents."
"The fact that Special Counsel Mueller’s report does not exonerate the president on a charge as serious as obstruction of justice demonstrates how urgent it is that the full report and underlying documentation be made public without any further delay," they wrote. "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."
The congressional leaders also took issues with the president's claims of being "exonerated."
On the campaign trail and on the Hill, Democrats made clear they are going to want every detail and document about the investigation and some have said they are willing to use their subpoena power in order to get it.
"It means make the request, if the request is denied subpoena, if the subpoenas are denied we will hold people before the Congress and yes, we will prosecute in court as necessary to get this information," House Intelligence Committee chairman Rep. Adam Schiff, D-Calif., said Sunday on "This Week".
Democratic candidates running to unseat President Donald Trump in 2020 reacted quickly to the news that Mueller delivered his report to DOJ with one consistent message: Make it public.
"The Mueller report must be made public — all of it," Sen. Kirsten Gillibrand, D-NY said in a presidential campaign speech outside the Trump International Hotel and Tower in New York City.
She added: "It is not often that I agree with Richard Nixon, but he was right to say that the American people have a right to know whether their president is a crook."
The much-awaited report was handed to the Justice Department for Barr’s review, and Congress was notified of the transfer late Friday afternoon, according to a Justice Department spokeswoman.
After reviewing Mueller's report, Barr will then send what he has described as his own "report" on the Mueller investigation to the top Democrats and Republicans on the House and Senate Judiciary committees. Barr has promised to be as transparent as possible, but it's unclear how extensive or detailed Barr's own "report" to Congress will be.
In a letter to the leaders of the House and Senate Judiciary committees, Barr wrote that he intends to "consult with Deputy Attorney General Rod Rosenstein and Special Counsel Mueller to determine what other information from the report can be released to Congress and the public consistent with the law."
A senior Department of Justice official told ABC News on Friday that the report would not include any further indictments.
Mueller and his team investigated how far the Kremlin went to interfere in the 2016 presidential election, including trying to determine whether any Americans may have helped those efforts.
At the heart of Mueller’s probe were two Russian operations: the spread of disinformation on social media, and the release of thousands of sensitive emails stolen by hackers from the Democratic National Committee and other Democratic targets. Mueller’s team has charged 25 Russian nationals and three foreign companies for their alleged role in those operations.
Trump and his Republican allies have derided the investigation as a “witch hunt.” But Rosenstein, FBI Director Chris Wray and Barr have each explicitly disputed that description.
At least four Trump associates, including Trump’s former national security adviser Michael Flynn, have pleaded guilty to lying to federal agents during the Russia-related investigation.
Another Trump associate, the president’s former attorney Michael Cohen, has pleaded guilty to lying to Congress about Trump’s business dealings in Moscow. And former Trump adviser Roger Stone has been charged with lying to Congress about his alleged role in tracking information stolen from Democrats during the 2016 presidential campaign.
Mueller’s investigation grew out of a probe the FBI launched in late July 2016.
By then, the FBI was already scrutinizing Trump campaign manager Paul Manafort’s business dealings with pro-Russian officials in Ukraine – dealings that have since landed Manafort in jail. And the FBI was keeping tabs on Trump adviser Carter Page, who was previously targeted for recruitment by Russian spies and had raised eyebrows with a trip to Moscow in mid-July 2016.
But claims by Trump adviser George Papadopoulos – that the Russians were touting “dirt” on Clinton – really set off alarms inside the FBI.
“If any Americans were part of helping the Russians [attack] us, that is a very big deal,” James Comey, who was FBI director at the time, later told lawmakers.
Several weeks after formally launching the Russia probe, counterintelligence agents leading the investigation in Washington received a so-called “dossier,” which had been compiled at the behest of Democrats and detailed uncorroborated allegations of coordination between Trump’s campaign and the Kremlin.
Some of the allegations involved Page, who was already on the FBI’s radar, so agents began secretly intercepting his communications. Page has never been charged with any crimes.
The wide-ranging investigation continued even after Trump took office. After Jeff Sessions became attorney general, he recused himself from oversight of the FBI’s Russia-related probe, citing his previous advocacy for Trump on the campaign trail.
Rosenstein subsequently assumed oversight of the investigation.
And then Trump shocked the federal law enforcement community: He fired Comey.
The move prompted Rosenstein to appoint Mueller to take over the whole matter, including a review of whether Comey's firing and other actions meant Trump improperly tried to obstruct the probe.
Comey later alleged that in a private meeting with Trump before his removal, the president directed Comey to “let [Flynn] go.”
Mueller has not released any evidence suggesting Trump committed a crime related to Russian efforts.
However, Mueller did uncover evidence of other possible crimes and referred those cases to other federal prosecutors.
The U.S. Attorney’s Office in Manhattan ended up tying Trump to federal campaign violations, alleging that – in the midst of the 2016 presidential campaign – Trump personally directed Cohen to silence two woman claiming affairs with Trump by making illegal payments to them; Trump has repeatedly denied the affairs.
Cohen has pleaded guilty for his role in the matter, but no other charges have been filed.
ABC News' Pierre Thomas, John Santucci and Kyra Phillips contributed to this report.