FBI boss Christopher Wray has robustly defended his staff, telling them "I stand with you", after President Trump approved the release of a memo accusing the Bureau of abuse of power and bias.
Drawn up by Republicans, it claims the FBI and the Department of Justice were biased against the President while investigating the claims that his election campaign colluded with Russia.
It alleges the FBI used unsubstantiated allegations by former British spy Christopher Steele to obtain a warrant to monitor a Trump campaign aide.
It also claims the Foreign Intelligence Surveillance Court was not told Mr Steele was demonstrably anti-Trump and that the research was funded by the Clinton campaign
The release of the previously classified memo has led to speculation that Mr Wray could resign after only six months in the job.
But in an internal note to the FBI's 35,000 staff he told them he was determined to stand up for the organisation.
"You've all been through a lot in the past nine months and I know it's often been unsettling, to say the least," wrote Mr Wray.
"And the past few days haven't done much to calm those waters… Let me be clear: I stand fully committed to our mission… I stand with you."
Despite not referencing the memo of the President specifically, Mr Wray told staff he was "determined to defend your integrity and professionalism every day", adding: "Talk is cheap; the work you do is what will endure".
Leading Democrats have warned of a "constitutional crisis" and told Donald Trump not to use the memo as a means of disrupting and discrediting the Russia investigation.
In a letter to the Persident, Democrats warned him against using the four-page document as a "pretext" to sack Deputy Attorney General Rod Rosenstein or Special Counsel Robert Mueller, who is overseeing the investigation.
"Firing Rod Rosenstein, DOJ (Department of Justice) leadership, or Bob Mueller could result in a constitutional crisis of the kind not seen since the Saturday night massacre," the Democrats wrote, referencing Richard Nixon's firing of the Watergate scandal prosecutor.
The White House insisted there had been no discussions about firing Mr Rosenstein.
The assurance comes despite reporters asking Mr Trump on Friday if he had confidence in the Deputy Attorney General and the President replying: "You figure that one out."
Speaking of the claims in the memo, Mr Trump said: "A lot of people should be ashamed."
"I think it's terrible," he said. "You want to know the truth. I think it's a disgrace. What's going on in this country, I think it's a disgrace."
White House Press Secretary Sarah Huckabee Sanders said the memo "raises serious concerns about the integrity of decisions made at the highest levels of the Department of Justice and the FBI to use the Government's most intrusive surveillance tools against American citizens".
The FBI, DOJ and Democrats – who say the memo uses cherry-picked classified details – had all pushed to stop its release, saying it could harm national security.
Democratic Leader Nancy Pelosi accused Mr Trump of "sending his friend Putin a bouquet", calling it a "desperate attempt to distract the American people from the truth about the Trump-Russia scandal".
Sky News US Correspondent Cordelia Lynch said a "big partisan political battle" was brewing.
"You now have the President at odds with the intelligence community, ploughing ahead despite the FBI's grave concerns and some central allegations within this memo that Republicans believe prove that the FBI was biased," she said.
Former FBI Director James Comey, who Mr Trump fired in May, savaged the decision to release the memo.
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He tweeted: "That’s it? Dishonest and misleading memo wrecked the House intel committee, destroyed trust with Intelligence Community, damaged relationship with FISA (Foreign Intelligence Surveillance) court, and inexcusably exposed classified investigation of an American citizen.
"For what? DOJ & FBI must keep doing their jobs."