WASHINGTON (Reuters) - U.S. Special Counsel Robert Mueller said on Wednesday his probe into Russian interference in the 2016 election was never going to end with criminal charges against President Donald Trump but he did not clear him and indicated it was up to Congress to decide whether he should be impeached.

In his first public comments since starting the investigation in May 2017, Mueller said Justice Department policy prevented him from bringing charges against a sitting president or filing sealed charges, telling reporters it was “not an option we would consider.”

But he also said his two-year investigation did not clear Trump of improper behavior and, while he did not use the word “impeachment,” he 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.

Congressional Democrats are debating whether to try to move ahead with impeachment in the Democratic-majority U.S. House of Representatives, even though the Republican-controlled Senate would be unlikely to complete the process outlined in the U.S. Constitution for removing a president from office by convicting him.

The White House and several top Republicans responded to Mueller’s comments on Wednesday by saying it was time to move on to other matters, while several candidates for the Democratic presidential nomination, called for impeachment.

One candidate, U.S. Senator Kamala Harris, wrote on Twitter: “What Robert Mueller basically did was return an impeachment referral.”

Calls to launch impeachment proceedings against Trump have grown among some Democrats, who have been frustrated by White House efforts to thwart congressional subpoenas seeking records and testimony related to the Russia investigation and other matters related to Trump and his family.

House Speaker Nancy Pelosi has been more cautious and is arguing that lawmakers should push ahead with their investigations before deciding whether to impeach Trump.

On Wednesday she said she was sticking with that plan.

“Nothing is off the table, but we do want to make such a compelling case, such an ironclad case, that even the Republican Senate, which at the time seems to be not an objective jury, will be convinced of the path that we have to take as a country,” she said at a San Francisco event.

Jerrold Nadler, the Democratic chairman of the House Judiciary Committee, said he would hold Trump “accountable” but declined to say whether he would bring impeachment charges.

“With respect to impeachment, all options are on the table and nothing should be ruled out,” he said at a news conference after Mueller’s appearance.

A redacted version of Mueller’s report was published in April. It concluded that Russia repeatedly interfered in the 2016 election and that Trump’s election campaign had multiple contacts with Russian officials, but did not establish a criminal conspiracy with Moscow to win the White House.

Mueller’s report declined to make a judgment on whether Trump obstructed justice, although it outlined 10 instances in which Trump tried to have Mueller fired or otherwise impede the investigation.

“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.”

Matthew Jacobs, a former federal prosecutor, said he thought Mueller was “saying in his own way that a crime was committed.”

A source close to Trump said the Mueller statement amounted to a “bad day for the home team.”

“Mueller’s statement today was a direct assault on the president,” said the source, who spoke on condition of anonymity. “And it will only empower the Democrats to be bolder and more aggressive in their move to impeach him.”

TRUMP DECLARES ‘CASE CLOSED’

Trump, who has repeatedly denounced Mueller’s investigation as a “witch hunt” and “hoax” meant to hobble his presidency, still took to Twitter to say the matter was settled.

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“Nothing changes from the Mueller Report,” he wrote. “There was insufficient evidence and therefore, in our Country, a person is innocent. The case is closed! Thank you.”

Mueller, a Republican who headed the FBI from 2001 to 2013, said he would not elaborate beyond what was contained in his 448-page report, signaling to Democrats that he was unlikely to provide them more ammunition for impeachment if he were to testify to a congressional committee.

Mueller, 74, said his office is formally closing its doors and he is now returning to life as a private citizen.

“Beyond what I’ve said here today and what is contained in our written work, I do not believe it is appropriate for me to speak further,” he said, adding that he would not go beyond what was in his report in any future testimony to Congress.

He did not take questions after making his statement.

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It was not clear whether Mueller would testify to Congress. He made clear he would prefer not to, although House Intelligence Committee Chairman Adam Schiff said he still expects to hear from the special counsel.

“While I understand his reluctance to answer hypotheticals or deviate from the carefully worded conclusions he drew on his charging decisions, there are, nevertheless, a great many questions he can answer that go beyond the report,” Schiff said.

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 offenses. “The ball is in our court, Congress,” he wrote on Twitter.

Mueller’s 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 7-1/2 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 Congress.

Since the report’s release, Democratic lawmakers have tried without success to get the unredacted version and underlying evidence.

Barr now is leading a review of the origins of the Russia investigation in what is the third known inquiry into the FBI’s handling of the matter. Trump harbors suspicions that the Democratic administration of President Barack Obama started the investigation in 2016 to undermine his presidency.

In attacking Mueller’s probe, Trump also has often attacked the integrity of the FBI and its investigators.

Mueller appeared to offer a response to that criticism on Wednesday.

The prosecutors, FBI agents, analysts and others who worked with him were “of the highest integrity,” he said.

He also defended the need to conduct the probe in the first place, saying Russia’s actions during the election campaign to interfere “needed to be investigated and understood.”