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".
FBI investigators have known about the existence of the newly discovered emails for weeks but did not inform the FBI director, according to US media reports.
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 8 November election, is being used for political purposes.
But Republican opponent Donald Trump has praised the FBI's decision.
Speaking at a rally in Nevada on Sunday, 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 - which the FBI said it discovered "in connection with an unrelated case" - contain or how significant they are to the investigation.
The unrelated case is said to be the inquiry into top Clinton aide Huma Abedin's estranged husband, former congressman Anthony Weiner, who is alleged to have sent sexually explicit emails to a 15-year-old girl in North Carolina.
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 Mr Comey's handling of the matter was "inappropriate" and the information provided 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."
Mrs Clinton's running mate Tim Kaine told NBC's Meet the Press the FBI director owed it to the public to be more forthcoming about the emails.
"We don't know whether they're to or from Hillary at all," the Virginia senator said. "[If he] 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."
But Mr Trump's running mate Mike Pence praised Mr Comey's decision, saying the emails showed Mrs Clinton was a "risky choice" and the Clinton campaign was practising the "old playbook of the politics of personal destruction" by "targeting the director of the FBI and questioning his personal integrity".
Mr Trump's campaign manager Kellyanne Conway told CNN that Mr Comey would have been accused of interfering in the election if he had not disclosed the newly discovered emails were under investigation.
How has the FBI probe affected the polls?
The bad news for Hillary Clinton is that the polls had already begun to tighten both nationally and in key battleground states before the FBI announcement on Friday.
Nationally, a new ABC News/Washington Post poll suggests Mrs Clinton is leading her rival by just one percentage point, down from a 12-point lead in the same poll a week ago. About a third of likely voters polled said they were less likely to support the Democrat after Mr Comey's disclosure.
But there is little evidence yet that the news will derail the former secretary of state's bid for the presidency.
In a new CBS poll of 13 battleground states, 52 percent of voters said they expected the emails to contain "more of what we already know" and most of those who said they were less likely to vote for Mrs Clinton were Republicans.