The White House says it is unaware whether the deputy attorney general threatened to quit after he was blamed for the FBI chief’s sacking.
Report says Rod Rosenstein was on the verge of resigning after the White House cast him as the prime catalyst to fire James Comey.
He detailed Mr Comey’s “serious mistakes” in a memo to President Donald Trump, just prior to the firing.
The sacking of Mr Comey has ignited a firestorm of criticism.
Democrats have called for a special prosecutor to take over the probe of alleged links between the Trump election team and Moscow, which Mr Comey was heading.
Mr Rosenstein reportedly made his threat unless the White House conveyed that the decision began with the president.
“I’m not aware of his threatening to resign,” White House Deputy Press Secretary Sarah Huckabee Sanders said.
She maintained that Mr Trump “very much had been thinking about letting Mr Comey go since November last year”.
Lawmakers in Washington are reeling after the White House abruptly removed Mr Comey on Tuesday for his handling of the inquiry over Hillary Clinton’s emails.
Critics accuse the Republican president of firing the nation’s top law enforcement official because he was leading the Russia inquiry.
Senior Democrats have also said they believed Mr Comey had recently asked the justice department for more resources for the FBI Trump-Russia investigation, which they say could have prompted his dismissal.
The White House has rejected the calls to appoint a special prosecutor to investigate allegations the Trump campaign colluded with the Kremlin over last year’s election.
Acting FBI director Andrew McCabe, the former deputy director, was due to testify before the Senate Intelligence Committee on Thursday on the panel’s annual hearing on worldwide threats.
The Senate Intelligence Committee has also invited Mr Comey to testify next week.
In a farewell letter tostaff, Mr Comey said he would not spend time on the decision or the way it was executed.
“I have long believed that a president can fire an FBI Director for any reason or for no reason at all,” he wrote.
President Trump defended his actions on Wednesday, saying Mr Comey was fired “because he was not doing a good job”. But Democratic senator Dianne Feinstein said she “understood” that Mr Comey had asked Mr Rosenstein for more resources for the FBI investigation.