US President Donald Trump's claim that he was "totally exonerated" by special counsel Robert Mueller has been rejected by Mr Mueller in testimony.
Testifying before Congress, Mr Mueller said he had not exonerated the president of obstruction of justice.
Mr Mueller spent two years investigating alleged collusion between the 2016 Trump campaign and Russia.
In his 448-page report he concluded that Russia interfered on Mr Trump's behalf but he did not establish a criminal conspiracy.
In all, 35 people and three companies were charged by the special counsel on matters relating both directly and indirectly to alleged Russian interference in the 2016 election. No members of the Trump family were charged.
The special counsel's office ruled that it was unable to charge the president with a crime, but could not exonerate him either. During his two-year investigation into Mr Trump, Mr Mueller rarely appeared in public and even more rarely issued statements.
In a hearing on Wednesday - the first of two consecutive Congressional committee hearings - Mr Mueller repeatedly declined to stray outside of confines of his 448-page report. He generally gave clipped answers or referred committee members back to the text of his findings.
The White House called Mr Mueller's testimony an "epic embarrassment for the Democrats"
Early on in his testimony, in response to questioning by Democratic committee chair Jerold Nadler, Mr Mueller confirmed that President Trump was incorrect to claim he had been "exonerated" by the special counsel.
Mr Mueller also confirmed that the special counsel team found evidence that the Russian government had attempted to interfere in the 2016 US election and did so with the intention of benefiting then-candidate Mr Trump.
"Over the course of my career I have seen a number of challenges to our democracy. The Russian government's effort to interfere in our election is among the most serious," he said.
Neither the rejection of Mr Trump's claim to have been exonerated nor the confirmation of Russian efforts to benefit Mr Trump were new revelations, but the power of having Mr Mueller state them categorically under oath are likely to be a boost for Democrats.
As the questioning went on, Mr Mueller appeared to be uncomfortable at finding himself in the middle of a partisan political battle between Democrat and Republican members - a situation he had made clear he wanted to avoid.
Republican members attacked the former special counsel's decision to neither accuse nor exonerate the president of obstruction of justice. Congressman John Ratcliffe said the decision held the president to a different legal standard than the established presumption of innocence.
Mr Ratcliffe said he agreed that the president should not be above the law, "but he damn sure shouldn't be below the law, which is where volume two of this report puts him".
Mr Mueller began to answer that the investigation into the president was a "unique situation" but was cut off by Mr Ratcliffe. The special counsel report outlines that there is a unique set of circumstances at play, because guidelines issued by the Department of Justice's Office of Legal Counsel (OLC) state that a sitting president cannot be indicted.
Ted Lieu, Democratic congressman for California, asked Mr Mueller: "The reason that you did not indict the president is because of OLC guidance that says you cannot indict a sitting president, correct?"
Mr Mueller replied: "Correct."
He later clarified that he did not intend to say he would have charged Mr Trump but for the OLC guidelines, clarifying that because of the guidelines he had decided not to reach a conclusion.
Democratic members of the committee focused on the detail of some of 10 occasions that the special counsel identified as cases in which the president may have obstructed justice. In particular they focused on the president's alleged attempt to have Mr Mueller fired.
President Trump's then-White House counsel Don McGahn gave extensive evidence to the special counsel investigation, telling Mr Mueller's team under oath that the president had asked him to fire the special counsel, but backed off after he refused.
In May, the New York Times reported that White House officials then asked Mr McGahn at least twice to state publicly that he had never believed the president obstructed justice.
For the president's critics, the McGahn episode represents the clearest and most damning case against the president for obstruction of justice.
Democratic members repeatedly attempted to draw Mr Mueller into a discussion about impeachment, which his report appears to refer to in a passage that reads: "The Constitution requires a process other than the criminal justice system to formally accuse a sitting President of wrongdoing."
But Mr Mueller appeared highly reluctant to confirm that the passage referred to impeachment, or to say whether or not his report was recommending the process, in light of the inability of the criminal justice system to bring charges against Mr Trump.
Mr Mueller said that the special counsel's office repeatedly requested an interview with the president but was refused, and no committee member asked why the president was not issued with a subpoena.
Mr Mueller also denied an allegation which President Trump has repeatedly made - that he was interviewed by the president for the role of FBI director, a job he had held previously for 12 years, the day before he was appointed special counsel. Mr Mueller said he had discussed recruitment for the role with the president but not as a candidate.
Prof Thaddeus Hoffmeister of University of Dayton School of Law told RNZ the one surprise was when Mr Mueller was asked whether or not the president could be indicted once he left office, and he said yes.
He said the Democrats' main purpose for calling Mr Mueller was so the US people could make their own judgements on whether the president acted appropriately - since few people had actually read the report.
- BBC / RNZ