In his first public comments since starting the investigation in May 2017, Mueller indicated it was up to Congress to decide if impeachment proceedings are justified.
He said Justice Department policy prevented him from bringing charges against a sitting president, telling reporters it was “not an option we would consider”.
But Mueller also said his two-year investigation did not clear Trump of improper behaviour, and pointed out there were other ways to hold presidents accountable.
“The Constitution requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing,” Mueller said as he announced his resignation from the Justice Department on Wednesday.
Democrats in Congress are debating whether to try to move ahead with impeachment, an effort that is almost certain to fall short in the Republican-controlled Senate.
“What Robert Mueller basically did was return an impeachment referral,” Senator Kamala Harris said on Twitter.
“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,” said Jerrold Nadler, the Democratic chairman of the House of Representatives Judiciary Committee.
A redacted version of Mueller’s report was published in April, concluding that Russia repeatedly interfered in the 2016 election and that Trump’s election campaign had multiple contacts with Russian officials.
However it did not establish a criminal conspiracy with Moscow to win the White House.
Mueller’s report also declined to make a judgement on whether Trump obstructed justice, although the report outlined 10 instances in which Trump tried to impede the investigation, including seeking to have Mueller fired.
“If we had confidence that the president clearly did not commit a crime, we would have said so,” Mueller said.
“We did not, however, make a determination as to whether the president did commit a crime.”
Trump, who has repeatedly denounced Mueller’s investigation as a “witch hunt” and “hoax” meant to hobble his presidency, said the matter was settled.
“Nothing changes from the Mueller Report,” he said on Twitter.
“There was insufficient evidence and therefore, in our Country, a person is innocent. The case is closed! Thank you.”
Mueller, who headed the FBI from 2001 to 2013, said he would not elaborate beyond what was contained in his 448-page report, signalling to Democrats that he was unlikely to provide them more ammunition for impeachment if he were to testify on Capitol Hill.
The House Judiciary Committee’s top Republican, Representative Doug Collins, said relitigating Mueller’s findings would only divide the country.
“It is time to move on from the investigation and start focusing on real solutions for the American people,” he said.
Only one Republican so far, Representative Justin Amash, has said Trump has committed impeachable offences.
“The ball is in our court, Congress,” he said on Twitter.
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