Sacked FBI deputy director Andrew McCabe has alleged he was being targeted because he is a crucial witness into whether President Donald Trump tried to obstruct the Russia investigation.
The US Attorney General Jeff Sessions announced Mr McCabe's dismissal on Friday, saying he felt justified in doing so after the Justice Department's internal watchdog found that the FBI deputy chief leaked information to reporters and misled investigators about his actions.
"The FBI expects every employee to adhere to the highest standards of honesty, integrity and accountability," Mr Sessions said.
But Mr McCabe, who played a crucial role in the bureau's investigations of Hillary Clinton and Russia's interference in the 2016 US presidential election, denied those claims and said he was facing retaliation by the Trump administration.
In a lengthy statement, Mr McCabe said he believed he was being politically targeted because he corroborated former FBI director James Comey's claims that Mr Trump tried to pressure him into killing the Russia probe.
Mr Trump sacked Mr Comey last year and acknowledged in a televised interview that it was because of "this Russia thing".
"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," Mr McCabe said.
"This attack on my credibility is one part of a larger effort … to taint the FBI, law enforcement, and intelligence professionals more generally."
The Republican president praised Mr McCabe's dismissal in a message on Twitter on Saturday.
"Andrew McCabe FIRED, a great day for the hard working men and women of the FBI - A great day for Democracy. 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!" Mr Trump wrote.
Mr McCabe was sacked two days before his 50th birthday, when he would have been eligible to retire from the Federal Bureau of Investigation with his full pension. The firing - which comes nine months after Mr Trump fired Mr Comey – puts Mr McCabe's pension in jeopardy.
He had stepped down from his position as FBI deputy director in January but remained on leave pending retirement.
His departure was triggered by a critical report from the Justice Department's inspector general that eventually led the FBI's Office of Professional Responsibility to recommend he be fired.
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Read more: Former FBI boss urges Department of Justice not to fire him days before he retires
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The report, which has yet to be made public, says Mr McCabe misled investigators about his communications with a former Wall Street Journal reporter who was writing about his role in probes tied to Ms Clinton, including an investigation of the Clinton family's charitable foundation.
In his statement, Mr McCabe denied ever misleading investigators.
He added that the release of the inspector general's report was "accelerated" after he testified behind closed doors before the US House Intelligence Committee where he revealed he could back up Mr Comey's claims. Mr Comey's firing has become central to questions about whether Mr Trump unlawfully sought to obstruct the Russia investigation.
Mr Comey's dismissal paved the way for Deputy Attorney General Rod Rosenstein to appoint special counsel Robert Mueller to lead the investigation into possible collusion between Mr Trump's campaign and Russia. Mr Trump has denied there was any collusion.
Mr McCabe could potentially be a crucial witness in Mr Mueller's investigation.
Mr Trump and other Republicans have accused Mr McCabe, a lifelong Republican who worked at the FBI for more than 20 years, of political bias and conflicts in connection with his oversight of investigations related to Ms Clinton, Mr Trump's Democratic rival in the 2016 election.
Some of that criticism stemmed from the fact that his wife, Jill McCabe, a Democrat, received donations for her unsuccessful 2015 Virginia state Senate campaign from Terry McAuliffe, who was then the state's governor and an ally of the Clintons.
Mr McCabe did not start overseeing the investigations until after his wife's campaign ended, the FBI has said, and therefore did not have a conflict of interest.
On Twitter last year, Mr Trump questioned why Mr McCabe was allowed to oversee an investigation into Ms Clinton's use of a private email server while his wife received donations from "Clinton puppets." He said Mr McCabe was "racing the clock to retire with full benefits".
Behind closed doors, Mr Trump also asked Mr McCabe who he voted for in the presidential election and referred to his wife as a loser, according to a source familiar with the matter.
Mr McCabe initially did not respond to the question but later told Mr Trump he did not vote in 2016, the source said.
Asked about this in January, Mr Trump said he did not recall asking Mr McCabe whom he voted for.
The inspector general's report is largely focused on how Mr McCabe answered questions about whether he leaked to the press in advance of a story that was critical of his oversight into the Clinton foundation investigation.
Mr McCabe contends he did not view this as a leak but as an authorised disclosure that is commonplace in Washington between reporters and government officials.
He said he answered questions truthfully, and later, when he felt investigators misunderstood his answers, he tried to clarify his responses with them.
