Donald Trump accused his "evil" rivals of doing "treasonous things" after he cleared of colluding with Russia during the presidential election in 2016.
The US president was speaking to reporters in the Oval Office in the aftermath of Robert Mueller's investigation, which found no proof of collusion between Trump's team and Russia.
President Trump did not say who he was referring to, but said: “There are a lot of people out there that have done some very, very evil things, very bad things.
“I would say treasonous things against our country.”
Mr Trump said “those people will certainly be looked at” and that he had been "looking at them for a long time".
Asked about the possibility of issuing pardons, Mr Trump said: “Haven’t thought about it.”
The president said the release of Mr Mueller’s report “wouldn’t bother me at all”.
He said he wished the inquiry could have been concluded more quickly, saying: “We can never let this happen to another president again.”
Asked whether Mr Mueller had acted honourably, Mr Trump, who was hosting Israeli leader Benjamin Netanyahu, responded: “Yes, he did.”
Mr Mueller also investigated whether Mr Trump obstructed justice but did not come to a definitive answer, attorney general William Barr said in a letter to Congress summarising Mr Mueller’s report.
The special counsel “does not exonerate” Mr Trump of obstructing justice, Mr Barr said, and his report “sets out evidence on both sides of the question”.
However, the president tweeted he had been “completely exonerated” of collusion and obstruction of justice.
Previously, speaking to reporters before boarding Air Force One to return to Washington from a weekend at his private club in Florida, Mr Trump said “it was a shame” the nation had to go through the investigation.
Mr Trump claimed the report found “there was no collusion with Russia, there was no obstruction”.
He also lashed out at the investigation, claiming without evidence that it was “an illegal takedown that failed”.
After consulting with other Justice Department officials, Mr Barr said he and deputy attorney general Rod Rosenstein determined the evidence “is not sufficient to establish that the president committed an obstruction of justice offence”.
Mr Barr released a four-page summary of Mr Mueller’s report on Sunday afternoon.
Mr Mueller wrapped up his investigation on Friday with no new indictments, bringing to a close a probe that has shadowed Mr Trump for nearly two years.
Mr Barr’s chief of staff called White House counsel Emmet Flood on Sunday to brief him on the report to Congress.
Mr Trump was at his Mar-a-Lago resort in Florida, about to return to Washington after spending the weekend there.
White House press secretary Sarah Sanders tweeted that Mr Trump had been completely exonerated.
Senate Judiciary Committee chairman Lindsey Graham added that “the cloud hanging over President Trump has been removed”.
Mr Graham, a close ally of Mr Trump, also said it is “a bad day for those hoping the Mueller investigation would take President Trump down”.
Republican Doug Collins said “there is no constitutional crisis” while House Majority Leader Kevin McCarthy, also a Republican said “it is time we move on for the good of the nation”.
Eric Trump, the President’s son, called for a “simple apology” from the media for “the hell everyone has been put through” during the two-year probe.
But the House Judiciary Committee chairman said Mr Mueller “clearly and explicitly is not exonerating the president”.
Democrat Jerry Nadler tweeted that Mr Barr’s letter to Congress says that while Mr Trump may have acted to obstruct justice, the government would need to prove that “beyond a reasonable doubt”.
But Mr Nadler tweeted that Congress must hear from Mr Barr about his decision making and see “all the underlying evidence for the American people to know all the facts”.
Top Democrats Nancy Pelosi and Chuck Schumer said Mr Barr is “not a neutral observer” and they urged the full release of the report.
Ms Pelosi, the House speaker, and Mr Schumer, the Senate Democratic leader, said that Mr Barr’s letter to Congress “raises as many questions as it answers”.
In a joint statement, the leaders said that Mr Barr’s past “bias” against the special counsel inquiry shows he is “not in a position to make objective determinations”.
They say that “the fact that Mueller’s report does not exonerate the president on a charge as serious as obstruction of justice demonstrates how urgent it is that the full report and underlying documentation be made public without any further delay”.
Several Democratic presidential candidates said Mr Mueller’s full report must be made public.
Kamala Harris said Mr Barr “must testify” before Congress, adding: “That is what transparency looks like.”
Elizabeth Warren, of Massachusetts, noted that politicians have voted to release the full Mueller report, not a summary from what she called Mr Trump’s “handpicked attorney general”.
New Jersey Senator Cory Booker said the public “deserves the full report and findings” immediately, “not just the in-house summary from a Trump Administration official”.
New York Senator Kirsten Gillibrand said the Mueller report “must be made public”. She tweeted: “The President works for the people, and he is not above the law.”
Mr Mueller’s investigation ensnared nearly three dozen people, senior Trump campaign operatives among them.
The probe illuminated Russia’s assault on the American political system, painted the Trump campaign as eager to exploit the release of hacked Democratic emails to hurt Democrat Hillary Clinton and exposed lies by Trump aides aimed at covering up their Russia-related contacts.
Mr Mueller submitted his report to Mr Barr instead of directly to Congress and the public because, unlike independent counsels such as Ken Starr in the case of President Bill Clinton, his investigation operated under the close supervision of the Justice Department, which appointed him.
Mr Mueller was assigned to the job in May 2017 by Mr Rosenstein, who oversaw much of his work.
Mr Barr and Mr Rosenstein analysed Mr Mueller’s report on Saturday, labouring to condense it into a summary letter of main conclusions.