Hillary Clinton's campaign has put pressure on FBI Director James Comey to release more details about the emails he says could be related to the investigation into her use of a private email server.
Tim Kaine, Mrs Clinton's running mate, said Mr Comey owed it to the public to be more forthcoming about the emails under review by the FBI with only 10 days remaining before the election.
Mr Kaine's message aimed to counter Republican rival Donald Trump, who has seized on the reignited email controversy in hopes of sowing fresh doubts about Mrs Clinton's trustworthiness.
"As far as we know now, Director Comey knows nothing about the content of these emails. We don't know whether they're to or from Hillary at all," Mr Kaine said.
He said if Mr Comey "hasn't seen the emails, I mean they need to make that completely plain. Then they should work to see the emails and release the circumstances of those once they have done that analysis".
Mrs Clinton's campaign chairman John Podesta said Mr Comey's handling of the matter was "inappropriate" and urged him to be more transparent.
The FBI is looking into whether there was classified information on a device belonging to Anthony Weiner, the disgraced ex-congressman who is separated from long-time Clinton aide Huma Abedin.
Mr Comey, in a letter to Congress on Friday, said the FBI had recently come upon new emails while pursuing an unrelated case and was reviewing whether they were classified.
Mrs Clinton's team says it is "unprecedented" for such FBI action so close to an election.
Her campaign has called on Mr Comey to release all the facts known so far.
The controversy over Mrs Clinton's email practices when she was Secretary of State has dogged her for more than a year.