Top aides to Democratic presidential candidate Hillary Clinton are angrily condemning FBI Director James Comey for revealing the bureau's probe of newly found emails related to her use of a private server.
They are demanding the quickly release of details about the discovery.
Sources familiar with the matter said on Sunday that Mr Comey likely has limited information about what is in the emails because the FBI lacks legal authority to examine their content thoroughly.
Clinton campaign chairman John Podesta and campaign manager Robby Mook blasted Mr Comey for sending 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," Podesta told CNN, while he accused the FBI chief of breaking precedent by disclosing aspects of an investigation so close to the election.
"Mr Comey really needs to come forward and explain why he took this unprecedented step, particularly when he said himself in the letter to the [Capitol] Hill that these may not even be significant," Mr Podesta said.
"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 Comey's letter, sent over the objections of Justice Department officials, plunged the final days of the White House race between Clinton and Republican Donald Trump into turmoil with Mr Trump focusing on the issue as proof for his argument that Ms Clinton is corrupt and untrustworthy.
Mr Comey, who announced in July that the FBI's long investigation of Ms Clinton's emails during her time as secretary of state 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.
The latest emails were discovered as part of a separate probe into Anthony Weiner, the estranged husband of Clinton aide Huma Abedin.
Mr Weiner, a former Democratic US congressman from New York, 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.
Sources familiar with the matter said FBI agents working on the Weiner investigation saw material on a laptop belonging to Mr Weiner that led them to believe it might be relevant to the investigation of Ms Clinton's email practices.
Mr Trump took to Twitter on Sunday to slam the Democrats as hypocrites for blasting Mr Comey.
"Hillary and the Dems loved and praised FBI Director Comey just a few days ago. Original evidence was overwhelming, should not have delayed!" Mr Trump said.
Ms Clinton had opened up a significant lead over Mr Trump in public opinion polls before the letter surfaced. The most recent polls had shown the lead narrowing slightly.