The head of the FBI has pledged to defend his agents against an attack on what he calls the bureau's independence.
US President Donald Trump has released a memo, which alleges bias against Republicans over the probe into possible Russian election interference.
Democrats warn the document is misleading and could harm national security.
FBI boss Christopher Wray has robustly defended his staff, telling them "I stand with you".
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.
Christopher A Wray, director of the Federal Bureau of Investigation (FBI) testifies before the United States Senate Committee Homeland Security and Governmental Affairs on "Threats to the Homeland" on Capitol Hill in Washington, DC in 2017 | Image: CNP/SIPA USA/PA Images
"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".
Former FBI Director James Comey, who Mr Trump fired in May, savaged the decision to release the memo.
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."
That’s it? Dishonest and misleading memo wrecked the House intel committee, destroyed trust with Intelligence Community, damaged relationship with FISA court, and inexcusably exposed classified investigation of an American citizen. For what? DOJ & FBI must keep doing their jobs.
— James Comey (@Comey) February 2, 2018
Leading Democrats have warned of a "constitutional crisis" and told Mr Trump not to use the memo as a means of disrupting and discrediting the Russia investigation.