President Donald Trump has fired back in a furious Twitter tirade Saturday after it was reported the FBI opened an investigation to determine whether he was a secret Russian agent in the days after he fired FBI Director James Comey.

Trump tweeted on Saturday morning: '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!

He continued: 'Funny thing about James Comey. Everybody wanted him fired, Republican and Democrat alike. After the rigged & botched Crooked Hillary investigation, where she was interviewed on July 4th Weekend, not recorded or sworn in, and where she said she didn’t know anything (a lie).'

Citing unnamed former law enforcement officials, the New York Times reported on Friday that the FBI opened the combined criminal and counterintelligence probe in May of 2017.

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President Donald Trump has fired back in a furious Twitter tirade Saturday after it was reported the FBI opened an investigation to determine whether he was a secret Russian agent in the days after he fired FBI Director James Comey

The FBI opened an investigation into President Donald Trump to determine whether he was a secret Russian agent in the days after he fired FBI Director James Comey, a new report claims

Trump continued tweeting: 'The FBI was in complete turmoil (see N.Y. Post) because of Comey’s poor leadership and the way he handled the Clinton mess (not to mention his usurpation of powers from the Justice Department). My firing of James Comey was a great day for America. He was a Crooked Cop.

'...who is being totally protected by his best friend, Bob Mueller, & the 13 Angry Democrats - leaking machines who have NO interest in going after the Real Collusion (and much more) by Crooked Hillary Clinton, her Campaign, and the Democratic National Committee. Just Watch!

'I have been FAR tougher on Russia than Obama, Bush or Clinton. Maybe tougher than any other President. At the same time, & as I have often said, getting along with Russia is a good thing, not a bad thing. I fully expect that someday we will have good relations with Russia again!

'Lyin’ James Comey, Andrew McCabe, Peter S and his lover, agent Lisa Page, & more, all disgraced and/or fired and caught in the act. These are just some of the losers that tried to do a number on your President. Part of the Witch Hunt. Remember the “insurance policy?” This is it!'

Trump's lawyer Rudy Giuliani had earlier dismissed the alleged investigation as baseless, pointing out that no evidence has publicly emerged in the 20 months since to support the idea that Trump is a secret Russian agent.

'The fact that it goes back a year and a half and nothing came of it that showed a breach of national security means they found nothing,' Giuliani told the Times.

Trump fired Comey on May 9, 2017, citing a letter from Deputy Attorney General Rod Rosenstein that sharply criticized Comey's handling of the Hillary Clinton email investigation.

However, sources tell the Times that two events raised alarm at the FBI that Trump had fired Comey in an attempt to stifle their investigation into possible Russia election meddling.

The first was a draft of the letter Trump wanted to send Comey informing him of the firing, which the Times reports mentioned the Russia investigation.

The draft was never sent, after Rosenstein purportedly urged Trump to drop mention of Russia in the letter.

The final version of the letter that was sent, and made public, does mention the investigation obliquely, thanking Comey for 'informing me, on three separate occasions, that I am not under investigation.'

Former FBI Director James Comey (C) speaks to the news media after a daylong closed door hearing at the US Capitol in Washington, DC in December 2018

The second issue that reportedly concerned the FBI was Trump's May 11, 2017 interview with NBC News, in which Lest Holt pressed him on the timing of the firing.

'I was going to fire Comey - there was no good time to do it,' Trump said.

'And in fact, when I decided to just do it, I said to myself - I said, you know, this Russia thing with Trump and Russia is a made-up story. It's an excuse by the Democrats for having lost an election that they should've won,' Trump continued.

Trump's detractors have taken this statement as proof that he fired Comey out of anger over the Russia investigation.

However, the President's supporters have said that he was indicating that he knew the timing of the firing might look bad, but decided to move forward with it anyhow because there was nothing behind the Russian collusion allegations.

Also fueling the FBI's concerns was Christopher Steele's 'dirty dossier' of unsubstantiated allegations, which had been funded by Democrats but at that point was justifying probes across the Justice Department.

Trump is seen in the May 11, 2017 with Lester Holt (right), which the FBI reportedly thought could be evidence that the President was a secret Russian agent

In response to the events, according to the Times, the FBI began investigating whether Trump was a secret Russian agent.

The investigation was one part criminal, into possible obstruction of justice, and one part counterintelligence, looking into possible national security concerns, the Times reports.

Comey's firing led Rosenstein, who had recommended the move, to appoint special counsel Robert Mueller to investigate any links or coordination between the Trump campaign and the Russian government.

Twenty months later, the Mueller probe has yet to publicly produce evidence of a conspiracy to tamper with the election. There are reports that the special counsel's probe could wrap up as soon as next month.