Robert Mueller said on Wednesday his two-year Russia investigation had not exonerated Donald Trump, but that he had lacked the power to charge a sitting president - passing the baton to Congress where a growing chorus is clamouring for impeachment.
Making his first public statement on the high-stakes probe into Moscow's interference in the 2016 election, the special counsel said longstanding Justice Department guidelines prevented him from charging Trump, despite at least 10 possible acts of criminal obstruction by the president.
But not indicting Trump was not equivalent to clearing him, Mr Mueller said, suggesting Congress has both the constitutional powers and the responsibility to pursue the case.
"If we had had confidence that the president clearly did not commit a crime, we would have said so," he said.
Mr Mueller's oblique allegation of serious wrongdoing sparked a new firestorm in Washington, with Trump again claiming vindication and a rising number of Democrats demanding to open impeachment proceedings.
"Nothing changes from the Mueller Report. There was insufficient evidence and therefore, in our Country, a person is innocent," the president tweeted minutes after Mr Mueller finished speaking.
"The case is closed! Thank you."
But Democratic Senator Elizabeth Warren was one of half a dozen White House hopefuls who seized upon Mr Mueller's remarks, to urge Congress to impeach Trump.
"Mueller leaves no doubt," she tweeted. "The Constitution leaves it up to Congress to act - and that's impeachment."
However, Jerry Nadler, the Democratic chairman of the House Judiciary Committee, which handles impeachment proceedings, said Mr Mueller made clear Trump 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.
Had 'no option' to charge the president
The taciturn Mr Mueller took to the podium to clarify his original conclusions more than two months after a summary of his report was first released by the Justice Department.
"Russian intelligence officers who were part of the Russian military launched a concerted attack on our political system," forming the basis for the investigation said the 74-year-old, long one of the most respected members of the Washington justice establishment.
Mr Mueller's 448-page report, finally released in redacted form on 18 April, detailed multiple contacts between the Trump campaign and Russia, including efforts to take advantage of Moscow's claimed possession of dirt on Trump's Democratic rival Hillary Clinton.
But in the end, Mr Mueller said, "There was insufficient evidence to charge a broader conspiracy" of collusion.
Special counsel Robert Mueller makes a statement about the investigation into Russian interference in the 2016 election at the Justice Department Source: AP
As for the obstruction side of the investigation, Mr Mueller explained that from the outset, charging Trump with any federal crime "was not an option that we could consider."
According to Justice Department policy, he said, "a president cannot be charged with a federal crime while he is in office. That is unconstitutional."
That left the public to parse the head-scratching, double-negative conclusion that Mr Mueller's team could not say that the president did not commit a crime.
Trump has assailed the Russia investigation as a "witch hunt" and a "hoax" that amounted to a "treasonous" attempt to overthrow him.
"The report was clear - there was no collusion, no conspiracy - and the Department of Justice confirmed there was no obstruction," said White House Spokeswoman Sarah Sanders.
"After two years, the special counsel is moving on with his life, and everyone else should do the same."
Pressure to impeach
A former director of the FBI, Mr 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 in 2017.
His appointment stunned Trump, who, White House witnesses told investigators, numerous times sought ways to undermine or fire Mr Mueller.
Mr Mueller's statement came 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, Mr Mueller 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.
Several candidates seeking the 2020 Democratic presidential nomination are now calling for the impeachment of Mr Trump.
"What Robert Mueller basically did was return an impeachment referral," California Senator Kamala Harris said in a tweet. "Now it is up to Congress to hold this president accountable.
"Mueller's statement makes clear what those who have read his report know: It is an impeachment referral, and it's up to Congress to act," said Massachusetts Senator Elizabeth Warren. "They should."
New Jersey Senator Cory Booker, another 2020 Democratic hopeful, said: "Mueller's statement makes it clear: Congress has a legal and moral obligation to begin impeachment proceedings immediately."
Former San Antonio Mayor Julian Castro, another Democrat seeking the 2020 nomination, said Mr Mueller "made clear this morning that his investigation now lays at the feet of Congress.
"There must be consequences, accountability, and justice," said former Texas congressman Beto O'Rourke. "The only way to ensure that is to begin impeachment proceedings."
Nancy Pelosi, the Democratic speaker of the House of Representatives, where impeachment would take place, appeared to be in no hurry to launch such proceedings against the Republican president.
Pressure on Pelosi
Mr Mueller made clear, though, that he thought it was up to Congress to pursue the case further.
"The constitution requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing," Mr Mueller said.
Mr Mueller's statement, and his announcement that he was resigning as special counsel to return to private life, piled heavy pressure on Ms Pelosi, the Democratic leader in Congress, to seriously consider launching the impeachment effort against Trump.
Ms Pelosi has fended off the idea, which would be fraught with political risk for the party 18 months ahead of presidential elections.
She said in a statement that Congress would step up investigations, while avoiding the word impeachment.
"The Congress holds sacred its constitutional responsibility to investigate and hold the President accountable for his abuse of power," she said.
"The American people must have the truth."
'No one is above the law'
Mr Nadler, Chairman of the House Judiciary Committee which would take the lead in any impeachment effort also avoided the "I" word while saying he would push ahead with a probe.
"Trump is lying. He is lying about the special counsel findings, lying about the testimony of key witnesses in the special counsel's report, and above all, lying and saying that the special counsel found no obstruction and no collusion," Mr Nadler told reporters.
"Therefore, as Mueller again highlighted this morning, it falls to Congress to respond to the crimes, lies and other wrongdoing of President Trump."
"We will do so. Make no mistake, no one, not even the president of the United States is above the law."
When asked whether that meant pushing ahead with impeachment, Mr Nadler replied: "All options are on the table and nothing should be ruled out."
