President Barack Obama believes FBI Director James Comey is a man of integrity and is not trying to influence the US presidential election by announcing scrutiny of additional emails linked to Democrat Hillary Clinton's private server, the White House has said.
White House spokesman Josh Earnest also said he has no "independent knowledge" of how Comey had arrived at his decision to make public the FBI email investigation or "what factors were considered" in his decision to discuss the issue publicly.
Earlier, a senior US Democrat has accused Mr Comey of breaking the law by trying to influence the election, as a source said investigators have secured a warrant to examine newly discovered emails relating to Hillary Clinton's private server.
The warrant will allow the Federal Bureau of Investigation to examine the emails to see if they are relevant to its probe of the private email server used for government work by Mrs Clinton while she was secretary of state from 2009 to 2013.
Mr Comey came under heavy pressure from Democrats to quickly provide details of the emails, as Clinton allies worried the prolonged controversy could extend beyond the 8 November election and cast a shadow over a Clinton transition if she wins the White House.
Mr Comey's disclosure of the email discovery in a letter to Congress on Friday plunged the final days of the White House race between the Democrat nominee and Republican Donald Trump into turmoil.
Mrs Clinton had opened a recent lead over Mr Trump in national polls, but it had been narrowing even before the email controversy resurfaced.
US Senate Democratic leader Harry Reid sent a letter to Mr Comey yesterday suggesting he violated the Hatch Act, which bars the use of a federal government position to influence an election.
"Through your partisan actions, you may have broken the law," Mr Reid, a senator from Nevada, said in the letter to Mr Comey.
Clinton campaign chairman John Podesta and campaign manager Robby Mook questioned Mr Comey's decision to send a letter notifying Congress of the email review before he even knew whether they were significant or relevant.
Mr Comey's letter was "long on innuendo, short on facts," Mr Podesta said on CNN's State of the Union, and accused the FBI chief of breaking precedent by disclosing aspects of an investigation so close to the election.
"We are calling on Mr Comey to come forward and explain what's at issue here," Mr Podesta said, adding the significance of the emails was unclear.
"He might have taken the first step of actually having looked at them before he did this in the middle of a presidential campaign, so close to the voting," Mr Podesta said.
Mr Comey's letter was sent over the objections of Justice Department officials.
But those officials did not try to stop the FBI from getting the warrant, a source familiar with the decision said, because they are interested in the FBI moving quickly on the probe.
Sources close to the investigation have said the latest emails were discovered as part of a separate probe of former Democratic US Representative Anthony Weiner, the estranged husband of Clinton aide Huma Abedin.
Mr Weiner is the target of an FBI investigation into illicit text messages he is alleged to have sent to a 15-year-old girl in North Carolina.
The FBI already had a warrant to search Mr Weiner's laptop in that inquiry, but needed a warrant to look at the material that might be related to Mrs Clinton.
Sources said FBI agents working on the Weiner investigation saw material on a laptop belonging to him that led them to believe it might be relevant to the investigation of Mrs Clinton's email practices.
Mr Trump has highlighted the issue as proof for his argument that Mrs Clinton is corrupt and untrustworthy.
"We have one ultimate check on Hillary's corruption and that is the power of the vote," Mr Trump told a rally in Las Vegas yesterday.
"The only way to beat the corruption is to show up and vote by the tens of millions."
Mr Comey, who announced in July that the FBI's long investigation of Mrs Clinton's emails was ending without any charges, said in his letter the agency would review the newly surfaced emails to determine their relevance to the investigation of her handling of classified information.
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