The FBI was advised by the US justice department not to inform Congress of a new inquiry into Hillary Clinton’s email use, officials say.
Justice department officials said the move would be inconsistent with rules designed to avoid the appearance of interference in an election.
FBI Director James Comey acted independently when he briefed lawmakers in a letter on Friday.
Mrs Clinton said the move was “unprecedented” and “deeply troubling”.
Leading Democratic senators have written to Mr Comey and to Attorney General Loretta Lynch urging them to provide more details about the investigation by Monday.
They argue that Mr Comey’s decision to reveal the reopening of the case, less than two weeks before the presidential election, is being used for political purposes.
But Republican opponent Donald Trump has praised the FBI’s decision.
Speaking at a rally in Phoenix on Saturday, Mr Trump accused the justice department of protecting the Democratic presidential candidate in a “rigged system”.
“The Department of Justice is trying their hardest to protect the criminal activity of Hillary Clinton,” Mr Trump said, offering no evidence for the assertion.
In his letter to Congress, Mr Comey said the FBI had learned of fresh emails which might be “pertinent” to its previous inquiry into Mrs Clinton’s use of a private server when she was secretary of state in the Obama administration.
Mr Comey, who has served in government under both Democratic and Republican presidents, has insisted that not making the inquiry public would be “misleading”.
It is not clear what the emails contain or how significant they are to the investigation.
Speaking to supporters in Florida on Saturday, Mrs Clinton said: “It’s not just strange, it’s unprecedented. And it is deeply troubling because voters deserve to get full and complete facts.
“So we’ve called on Director Comey to explain everything right away, put it all out on the table.”
Mrs Clinton has said she is confident the investigation into the emails will not change the FBI’s original finding in July, which criticised her but cleared her of any illegal acts.
Clinton campaign chairman John Podesta said the information provided by Mr Comey was “long on innuendo” and “short on facts”.
There was, he said, “no evidence of wrongdoing. No charge of wrongdoing. No indication this is even about Hillary”.