Former FBI deputy director Andrew McCabe took notes about interactions with President Donald Trump and the notes were turned over to special counsel Robert Mueller’s team some time ago, a source with knowledge of the situation confirmed to NBC News on Saturday.

McCabe was fired by Attorney General Jeff Sessions late Friday, two days before he was to retire and become eligible for full pension benefits.

Mueller is investigating Russian interference in the 2016 presidential election and whether there was any coordination with anyone connected to Trump’s campaign, and also possible obstruction of justice.

The Wall Street Journal reported earlier Saturday that McCabe took notes of conversations with the president and that those notes were turned over to Mueller. Axios and the Associated Press also earlier Saturday reported that memos had been turned over to Mueller.

Trump again attacked McCabe on Twitter Saturday, calling his firing “A great day for Democracy” and saying "Sanctimonious James Comey was his boss and made McCabe look like a choirboy. He knew all about the lies and corruption going on at the highest levels of the FBI!"

Comey, the FBI director whose firing by Trump in May 2017 in part led to Mueller being appointed as special counsel, fired back in a tweet of his own.

Mr. President, the American people will hear my story very soon. And they can judge for themselves who is honorable and who is not. — James Comey (@Comey) March 17, 2018

"Mr. President, the American people will hear my story very soon. And they can judge for themselves who is honorable and who is not," Comey said in the tweet.

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Comey also wrote memos of his interactions with Trump. He testified in a Senate committee hearing last June that he wrote the memos because he was concerned the president might lie about the nature of their meetings.

Related: Trump's attorney John Dowd calls for end to Mueller probe

Comey also admitted to asking a friend to leak some memos to the media, after Trump tweeted that Comey had "better hope that there are no 'tapes' of our conversations before he starts leaking to the press!"

Trump tweeted again early Sunday morning, suggesting the pair's notes should be called "Fake Memos."

Wow, watch Comey lie under oath to Senator G when asked “have you ever been an anonymous source...or known someone else to be an anonymous source...?” He said strongly “never, no.” He lied as shown clearly on @foxandfriends. — Donald J. Trump (@realDonaldTrump) March 18, 2018

Spent very little time with Andrew McCabe, but he never took notes when he was with me. I don’t believe he made memos except to help his own agenda, probably at a later date. Same with lying James Comey. Can we call them Fake Memos? — Donald J. Trump (@realDonaldTrump) March 18, 2018

Trump has repeatedly denied any collusion with the Russian effort to interfere in the presidential election. Trump on Twitter pointed to the release by Republican members of the House Intelligence Committee which said they found no evidence Russians colluded with any members of the Trump campaign.

The report from the GOP members of the House committee has been criticized as partisan. It also disputed a key finding from the intelligence community that Russia had developed a preference for the Republican nominee during the election. Democrats on the House committee called the Republicans' move to end the probe a "capitulation" to the White House.

A January 2017 assessment from intelligence agencies found with "high confidence" that Russian President Vladimir Putin and his government expressed a "clear preference" for Donald Trump in the campaign, and sought to influence the election on his behalf as part of Putin's "longstanding desire to undermine the U.S.-led liberal democratic order."

Related: McCabe: Trump wants to destroy me to stop Mueller probe

McCabe's firing and Trump's response early Saturday sparked outrage from Democrats and former members of the intelligence community who have cast the decision to remove McCabe as something akin to a political hit-job.

John Brennan, the former director of the CIA under President Barack Obama and former Director of the National Counterterrorism Center under President George W. Bush, slammed what he described as Trump's "venality, moral turpitude, and political corruption."

When the full extent of your venality, moral turpitude, and political corruption becomes known, you will take your rightful place as a disgraced demagogue in the dustbin of history. You may scapegoat Andy McCabe, but you will not destroy America...America will triumph over you. https://t.co/uKppoDbduj — John O. Brennan (@JohnBrennan) March 17, 2018

In firing McCabe, Sessions was accepting the recommendation of the FBI's Office of Professional Responsibility, which proposed the termination based on the findings of the Justice Department's inspector general. That office has been examining the bureau's handling of the Clinton e-mail investigation.

Sessions said in a statement Friday that McCabe’s firing came "after an extensive and fair investigation." He said that "Both the OIG and FBI OPR reports concluded that Mr. McCabe had made an unauthorized disclosure to the news media and lacked candor — including under oath — on multiple occasions."

McCabe in a statement said "I am being singled out and treated this way because of the role I played, the actions I took, and the events I witnessed in the aftermath of the firing of James Comey."

Related: Read former FBI deputy director Andrew McCabe's full statement after firing

"This attack on my credibility is one part of a larger effort not just to slander me personally, but to taint the FBI, law enforcement, and intelligence professionals more generally," McCabe said. "It is part of this Administration's ongoing war on the FBI and the efforts of the Special Counsel investigation, which continue to this day.

McCabe, who has served with the FBI for 21 years, turns 50 on Sunday, which would have made him eligible for certain substantial retirement benefits. His firing puts his federal pension in jeopardy.