US Special Counsel Robert Mueller has made his first public comments on his investigation of Russian interference in the 2016 US presidential election.
He said charging President Donald Trump with a crime was "not an option".
He said he is formally closing the Special Counsel's office, and he is resigning from the Justice Department and returning to private life.
Mr Mueller did not take questions after his statement at the Justice Department.
A redacted version of the 448-page Mueller report was published in April, concluding the campaign of President Trump did not engage in a criminal conspiracy with Russia to secure his victory.
Mr Mueller declined to make a judgment on whether Mr Trump obstructed justice, although the report outlined ten instances in which Mr Trump tried to impede the investigation.
The investigation ensnared dozens of people, including several top Trump advisers and a series of Russian nationals and companies.
Among them are his former campaign chairman, Paul Manafort, who is serving seven-and-a-half years in prison for financial crimes and lobbying violations, and his former personal lawyer, Michael Cohen, who recently began a three-year sentence for campaign-finance violations and lying to the US Congress.
Mr Mueller emphasised that he was bound by the longstanding policy that a sitting president cannot be charged with a crime, saying it was now up to Congress, by its impeachment powers, to pursue the issue.
"If we had had confidence that the president clearly did not commit a crime, we would have said so," he stressed.
President Trump, who has described the Russia investigation as a treasonous "witch hunt" and a "hoax," declared the case over minutes after Mr Mueller finished speaking.
"Nothing changes from the Mueller Report. There was insufficient evidence and therefore, in our Country, a person is innocent," he tweeted. "The case is closed! Thank you".
Nothing changes from the Mueller Report. There was insufficient evidence and therefore, in our Country, a person is innocent. The case is closed! Thank you.
— Donald J. Trump (@realDonaldTrump) May 29, 2019
However, Jerry Nadler, the Democratic chairman of the House of Representatives Judiciary Committee, which handles impeachment proceedings, said Mr Mueller made it clear the US President was "lying" about the report's conclusions.
"Given that Special Counsel Mueller was unable to pursue criminal charges against the President, it falls to Congress to respond to the crimes, lies and other wrongdoing of President Trump - and we will do so," Mr Nadler said in a statement.
Given that Special Counsel Mueller was unable to pursue criminal charges against the President, it falls to Congress to respond to the crimes, lies and other wrongdoing of President Trump – and we will do so. No one, not even the President of the United States, is above the law. https://t.co/w61a8rRQeK
— (((Rep. Nadler))) (@RepJerryNadler) May 29, 2019
A former director of the FBI, Robert Mueller was rarely seen and never heard from as he conducted the Russia investigation in utter secrecy after being named to lead it on 17 May 2017.
His appointment stunned President Trump, who, White House witnesses told investigators, numerous times sought ways to undermine or fire Mr Mueller.
Mr Mueller's statement comes as Democrats in Congress are pressing for his testimony on the Russia investigation as possible support for an impeachment effort against the president.
Indicating he does not wish to testify, he argued that the investigation's final report is the Special Counsel's Office's "final position," and that it would be inappropriate to speak further about it.
"We will not comment on any other conclusions or hypotheticals about the president," he said. "I would not provide information beyond that which is already public in any appearance before Congress."
"I hope and expect this to be the only time that I will speak about this matter.
"I am making that decision myself, no one has told me whether I can or should testify or speak further about this matter," he said.
Mr Mueller wrapped up by reiterating what he called "the central allegation of our indictments, that there were multiple, systematic efforts to interfere in our election.
"That allegation deserves the attention of every American," he said.
