Democrats demanded to receive Mueller's entire report and his underlying evidence to further their own multiple investigations into the president.
In a sprawling investigation spanning almost two years, Special Counsel Robert Mueller examined two burning questions: Did President Donald Trump or any of his associates conspire with Russia's efforts to interfere in the 2016 presidential election and did Trump commit obstruction of justice during the investigation?
Trump was in his private quarters at Mar-a-Lago when staff and his attorneys described Barr's letter about Mueller's findings.
The Democratic heads of six House committees wrote a joint letter to Barr Friday, saying, "If the Special Counsel has reason to believe that the president has engaged in criminal or other serious misconduct, then the Justice Department has an obligation not to hide such information".
US President Donald Trump declared Sunday that he had been completely exonerated, after his campaign was cleared of colluding with Russian Federation in the 2016 election campaign.
"The special counsel did not find any collusion and did not find any obstruction", White House spokeswoman Sarah Sanders said in a tweet.
Barr said he hoped to make public a summary of its "principal conclusions" over the weekend and a person familiar with the matter said it was expected to come out on Sunday.
Mr Mueller, 74, found that Russian Federation sought to influence the election through misinformation pumped out on social media by a "troll farm" based in St Petersburg and by stealing and releasing emails from senior Democrats.
Democrats nearly immediately raised questions about the report's finding about obstruction.
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"It's one of those hard realities that sometimes things don't turn out the way you wanted them to", said Democratic political consultant Colin Strother, who is based in Austin, Texas.
"Seems like the Department of Justice is putting matters squarely in Congress' court", tweeted Democratic Representative Jerry Nadler, the chairman of the House Judiciary Committee. "To be honest it's a shame that your president has had to go through this", Trump said. Russian Federation says it did not interfere in the election, although USA intelligence agencies concluded that it did.
The headline finding was that no "Trump campaign official or associate conspired or knowingly co-ordinated" with operatives connected to the Russian government during the presidential campaign.
Mr Trump faces continuing risk from other investigations, with federal prosecutors in NY looking into his company, presidential campaign and inaugural committee.
Over the course of the investigation, Mueller got convictions or guilty pleas from Trump's campaign chairman, deputy campaign chairman, national security advisor and longtime personal attorney and indicted his longtime confidant, Roger Stone.
Trump took White House counsel Pat Cipollone and Emmet Flood with him to Florida. "America is the greatest place on earth", he told reporters gathered on the South Lawn.
Spin-off investigations were opened by other federal prosecutors into Trump's hush money payments to a porn star, his inauguration committee's fundraising and his business.
If the full Mueller report, and underlying investigative evidence, is not turned over to Congress, Democrats who control the House have vowed to subpoena it and possibly call Mueller to testify about his findings.