James Comey told politicians days before he was sacked that he had asked the Justice Department for more resources to pursue the FBI's investigation into Russia's interference in last year's presidential election, three US officials have said.
The officials said Mr Comey met last week with Rod Rosenstein, the deputy attorney general, to make the request.
Mr Comey then alerted politicians with ties to the concurrent congressional investigations into Russia's meddling, according to the officials.
Justice Department spokeswoman Sarah Isgur Flores said it was false that Mr Comey had asked Mr Rosenstein for money for the Russia investigation.
The revelations raise new questions about what prompted President Donald Trump's decision to fire Mr Comey.
The White House has cited a memo from Mr Rosenstein, in which he criticises Mr Comey's handling of last year's investigation into Democrat Hillary Clinton's email practices.
Mr Rosenstein's memo makes no mention of the FBI's Russia investigation, which is probing both Russia's hacking of Democratic groups last year and whether Trump campaign associates had ties to Moscow's election interference.
Mr Trump defended his decision on Wednesday, asserting in a flurry of tweets that both Democrats and Republicans "will be thanking me" for his action.
He did not mention any effect the dismissal might have on the FBI and congressional investigations into contacts between his 2016 election campaign and Russia.
James Comey will be replaced by someone who will do a far better job, bringing back the spirit and prestige of the FBI.— Donald J. Trump (@realDonaldTrump) May 10, 2017
"He wasn't doing a good job. Very simply. He was not doing a good job," Mr Trump said in brief remarks to reporters in the Oval Office, where he was joined by former secretary of state Henry Kissinger.
The abrupt firing of Mr Comey threw into question the future of the FBI's investigation and immediately raised suspicions of an underhand effort to stymie a probe that has overshadowed the administration from the outset.
Mr Trump has ridiculed the investigations as "a hoax" and denied any campaign involvement with the Russians.
Democrats compared Mr Comey's removal to President Richard Nixon's Saturday Night Massacre during the Watergate investigation and renewed calls for the appointment of a special prosecutor.
Mr Kissinger, who was meeting Mr Trump, was Mr Nixon's secretary of state in 1973, having just moved over from being his national security adviser.
Earlier on Wednesday, Senate Democratic leader Chuck Schumer urged Attorney General Jeff Sessions and his deputy, Mr Rosenstein, to appear before the Senate to answer questions about the circumstances surrounding Mr Trump's action.
However, Senate Majority Leader Mitch McConnell brushed aside calls for a special prosecutor, saying a new investigation into Russian meddling would only "impede the current work being done".
.@realDonaldTrump tells journalists he sacked ex-FBI chief James Comey because "he wasn't doing a good job" https://t.co/VYqICcJOBw pic.twitter.com/YB8znwGEp9— BBC News (World) (@BBCWorld) May 10, 2017
He noted that Democrats had repeatedly criticised Mr Comey in the past and some had called for his removal.
Mr Trump made a similar case on Twitter, saying Mr Comey had "lost the confidence of almost everyone in Washington", adding: "When things calm down, they will be thanking me!"
Vice President Mike Pence said at the Capitol that Mr Trump had made "the right decision at the right time".
The Justice Department said Mr Sessions was interviewing candidates to serve as an interim replacement.
Mr Comey's deputy, FBI veteran Andrew McCabe, became acting director after Mr Comey was fired.
In his brief letter on Tuesday to Mr Comey, Mr Trump said the firing was necessary to restore "public trust and confidence" in the FBI.
The administration paired the letter with a scathing review by Mr Rosenstein, the recently confirmed deputy attorney general, of how Mr Comey handled the investigation into Mrs Clinton's email practices, including his decision to hold a news conference announcing its findings and releasing "derogatory information" about Mrs Clinton.
"You can't make this stuff up," Representative Adam Schiff, the top Democrat on the House Intelligence Committee, said on MSNBC.
As to Mr Trump's contention that the firing had to do with Mr Comey's decision-making on such matters as the Clinton emails, he said: "Nobody believes that."
Mr Trump, in his letter, pointedly thanked Mr Comey for telling him three times "that I am not under investigation".
The FBI has not confirmed that Mr Comey ever made those assurances to the president.
In public hearings, Mr Comey has declined to answer when asked if Mr Trump is under investigation, urging politicians not to read anything into that statement.
While Mr Comey has drawn anger from Democrats since he reopened the email investigation in the closing days of last year's campaign, they did not buy that justification for his firing.
Several Republicans joined them in raising concerns over how it could affect probes into possible co-ordination between Trump associates and Russia to influence the election.
In one of the strongest statements by Republicans, Senator Richard Burr, chairman of the Senate Intelligence Committee, said: "I am troubled by the timing and reasoning of Director Comey's termination.
"His dismissal further confuses an already difficult investigation by the committee."
Mr Schumer told Mr Trump in a phone call he thought removing Mr Comey was a mistake.
On Wednesday, Mr Trump labelled the Senate minority leader "Cryin' Chuck Schumer".
Mr Trump will now appoint a successor at the FBI, which has been investigating since late July, who will almost certainly have an impact on how the probe moves forward and whether the public will accept its outcome.
It was only the second firing of an FBI director in history.
President Bill Clinton dismissed William Sessions amid allegations of ethical lapses in 1993.
Republican Senator John McCain said Congress must form a special committee to investigate Russia's interference in the election.
Mr Comey, 56, was nominated by President Barack Obama for the FBI post in 2013 to a 10-year term, though that appointment does not ensure a director will serve the full term.
Praised frequently by both parties for his independence and integrity, he spent three decades in law enforcement.
Before the recent controversies, the former deputy attorney general in the George W Bush administration was perhaps best known for a remarkable 2004 stand-off with top officials over a federal domestic surveillance programme.
In March of that year, Mr Comey rushed to the hospital bed of attorney general John Ashcroft to physically stop White House officials in their bid to get his ailing boss to reauthorise a secret no-warrant wiretapping programme.
But his prominent role in the 2016 presidential campaign raised questions about his judgment and impartiality.
Though the FBI did not recommend charges against Mrs Clinton for mishandling classified information, Mr Comey was blisteringly critical of her decision to use a personal email account and private internet server during her four years as secretary of state.
Mr Comey strongly defended his decisions during a Senate Judiciary Committee hearing last week.
He said he was "mildly nauseous" at the thought of having swayed the election but also said he would do the same again.
Mrs Clinton has partially blamed her loss on Mr Comey's disclosure to Congress less than two weeks before Election Day that the email investigation would be revisited.
Mr Comey later said the FBI, again, had found no reason to bring any charges.