President Donald Trump blasted the FBI on Saturday, insisting it acted "for no reason & with no proof" when it opened an investigation into whether he was acting on Russia's behalf after he fired the agency's director, James Comey, in May 2017.
TheNew York Times reported that the FBI launched the previously undisclosed counterintelligence investigation to determine whether Mr Trump posed a national security threat, at the same time that it opened a criminal probe into possible obstruction of justice by the president.
The FBI investigation was subsequently folded into the broader probe by Special Counsel Robert Mueller into Russia's meddling in the 2016 election and possible collaboration by the Trump campaign.
No evidence has publicly emerged that President Trump was secretly in contact with or took direction from Russian officials, the Times said.
"Wow, just learned in the Failing New York Times that the corrupt former leaders of the FBI, almost all fired or forced to leave the agency for some very bad reasons, opened up an investigation on me, for no reason & with no proof, after I fired Lyin' James Comey, a total sleaze!" President Trump tweeted.
According to President Trump, "the FBI was in complete turmoil... because of Comey's poor leadership" and the way he handled the investigation into Hillary Clinton's use of a private server to send some government emails.
"My firing of James Comey was a great day for America," he said, describing the former FBI director as "a Crooked Cop who is being totally protected by his best friend, Bob Mueller."
Such standard reactions from Mr Trump "do nothing to address the incredibly serious nature of these allegations," said Democratic Congressman Jerrold Nadler, Chairman of the House Judiciary Committee.
"There is no reason to doubt the seriousness or professionalism of the FBI," Mr Nadler said in a statement which said his committee "will take steps to better understand both the president's actions and the FBI's response to that behaviour, and to make certain that these career investigators are protected from President Trump's increasingly unhinged attacks."
The Times said that the FBI had been suspicious of Mr Trump's ties to Russia during the 2016 campaign.
But it held off on opening an investigation until the president sacked Mr Comey, who refused to pledge allegiance to Mr Trump and roll back the nascent Russia investigation.
The President has repeatedly criticised the Mueller probe as a "witch hunt" and views it as an attempt to besmirch the legitimacy of his presidency.
Secretary of State Mike Pompeo, who was CIA director at the time the investigation was launched, declined to comment on The New York Times report, but insisted in an interview with CBS that "the notion that President Trump is a threat to American national security is absolutely ludicrous."
Mr Mueller has indicted 33 people, including members of Russia's GRU military intelligence, and chalked up convictions against some of the president's close associates.
President Trump's ex-national security advisor, Michael Flynn, pleaded guilty to lying to investigators about his Moscow ties.
His former personal lawyer, Michael Cohen, has been sentenced to three years in prison for multiple crimes, including felony violations of campaign finance laws that prosecutors allege were carried out under Mr Trump's direction.
Mr Turmp's former presidential campaign chairman, Paul Manafort, has been convicted in one case brought by Mr Mueller and pleaded guilty in another, over financial crimes related to his work in Ukraine before the 2016 campaign, and for witness tampering.
Cell phone records show that Mr Cohen was near Prague during the summer of 2016, supporting claims that he met there with Russian officials during the presidential election campaign, McClatchy news service has reported.
Mr Cohen, who will testify in Congress on February 7, insists that he has never been to Prague, but added in a tweet: "#Mueller knows everything!"
Mr Manafort has admitted to sharing polling data with a Russian during the 2016 presidential race, according to a court filing inadvertently made public by his lawyers. CNN reported that the intended recipients were two pro-Russia Ukrainian oligarchs.
Mr Manafort has denied lying to investigators about his dealings with the Russian, Konstantin Kilimnik, a political consultant with alleged intelligence ties, claiming he merely forgot details during the hectic campaign.
The specifics of the Mueller allegations were not previously known publicly, having been blacked out in a heavily redacted December 7 filing by the prosecutor's team.
But in Mr Manafort's response this month, the electronic formatting for the redaction could easily be bypassed, revealing exactly what he was accused of lying about.