President Donald Trump on Sunday questioned the impartiality of special counsel Robert Mueller's investigation and raised doubts about whether a fired top FBI official kept personal memos outlining his interactions with Trump.

Trump, in a series of tweets, elevated his simmering grievances to a boil against Mueller, whose team is examining Trump campaign ties to Russia and possible obstruction of justice; onetime FBI deputy director Andrew McCabe, who was dismissed Friday by the attorney general; and former FBI Director James Comey, ousted last year by Trump.

The president's Twitter barbs follow closely on the call by Trump's personal lawyer for the Trump-appointed No. 2 Justice Department official overseeing Mueller's inquiry to "bring an end" to that investigation. And Republicans on the House Intelligence Committee said this past week that they had completed a draft report concluding, after a yearlong investigation, that there was no collusion or coordination between the Trump campaign and Russia; committee Democrats vehemently disagreed.

Trump asserted that Mueller's team of investigators has a large number of "hardened Democrats, some big Crooked Hillary supporters, and Zero Republicans? ... does anyone think this is fair? And yet, there is NO COLLUSION!"

It is true that some Mueller investigators have contributed to Democratic political candidates, including Trump's 2016 opponent, Hillary Clinton. But Justice Department policy and federal service law bar discrimination in the hiring of career positions on the basis of political affiliation, and experts say there is no rule barring such donations.

Mueller, a Republican himself, was appointed FBI director by a GOP president, George W. Bush. Deputy Attorney General Rod Rosenstein, an ex-U.S. attorney under Bush and Democratic President Barack Obama, was named to the Justice Department post by Trump and put in charge of Mueller's investigation by Attorney General Jeff Sessions, a former Republican senator tapped by Trump for his Cabinet. Sessions stepped aside from overseeing the investigation after the Justice Department acknowledged he had spoken twice with the Russian ambassador in 2016 and had failed to disclose the contacts during his Senate confirmation process.

Trump may have felt more emboldened when Sessions, acting on the recommendation of FBI disciplinary officials, sacked McCabe on Friday, two days before McCabe's retirement date. "A great day for Democracy," Trump tweeted afterward and asserted without elaboration that McCabe, whom the president has long scorned, knew "all about the lies and corruption going on at the highest levels off the FBI!"

The Associated Press later reported that McCabe kept personal memos detailing interactions with the president that have been provided to Mueller's office and are similar to notes compiled by Comey.

A skeptical Trump tweeted: "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?"

It wouldn't be unusual for a senior official to make notes soon after meeting with the president.

McCabe's memos include details of his interactions with the president, according to a person with direct knowledge of the situation who wasn't authorized to discuss the notes publicly and spoke on condition of anonymity. The memos also recount different conversations he had with Comey, who kept notes on meetings with Trump that unnerved him.

Comey was a target Sunday when Trump claimed that Comey lied under oath at a Senate hearing in 2017, shortly before his firing, when he said he had never been an anonymous source. Comey, who is releasing a book next month, tweeted on Saturday in response to McCabe's firing: "Mr. President, the American people will hear my story very soon. And they can judge for themselves who is honorable and who is not."

The precise contents of McCabe's memos are unknown, but they possibly could help substantiate McCabe's assertion that he was unfairly maligned by a White House he says had declared "war" on the FBI and Mueller's investigation. They almost certainly contain, as Comey's memos did, previously undisclosed details about encounters between the Trump administration and FBI that could be of interest to Mueller.

Sessions said he dismissed McCabe on the recommendation of FBI disciplinary officials who said McCabe had not been candid with a watchdog office investigation. An upcoming inspector general's report is expected to conclude that McCabe, who spent more than 20 years with the FBI, had authorized the release of information to the media and was not forthcoming with the watchdog office as it examined the bureau's handling of an investigation into Clinton's emails.

McCabe has vigorously disputed the allegations and said his credibility had been attacked as "part of a larger effort not just to slander me personally" but also the FBI and law enforcement.

"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," he added. "Their persistence in this campaign only highlights the importance of the special counsel's work."

Also over the week, Trump's personal lawyer, John Dowd, cited the "brilliant and courageous example" by Sessions and the FBI's Office of Professional Responsibility and said Rosenstein should "bring an end" to the Russia investigation "manufactured" by Comey.

Dowd told the AP that he neither was calling on Rosenstein to fire the special counsel immediately nor had discussed with Rosenstein the idea of dismissing Mueller or ending the probe.

Mueller is investigating whether Trump's actions, including Comey's ouster, constitute obstruction of justice. McCabe could be an important witness, and his memos could be used by investigators as they look into whether Trump sought to thwart the FBI probe. Comey's own memos, including one in which he says Trump encouraged him to end an investigation into former national security adviser Michael Flynn, have been provided to Mueller and are part of his investigation.

McCabe asserted he was singled out by the administration because of the "role I played, the actions I took, and the events I witnessed in the aftermath" of Comey's firing last May.

He became acting director after that but clashed with the Trump administration, including when he publicly rejected White House assertions that Comey had lost the support of the rank and file. He abruptly left the deputy director position in January and went on leave.