Democratic presidential candidate Hillary Clinton on Saturday called the timing of the FBI's announcement that it was assessing new evidence in her email case "unprecedented" and "deeply troubling."
At a campaign appearance in Daytona Beach, Florida, Clinton told her audience, "It's pretty strange to put something like that out with such little information, right before an election."
FBI Director James Comey said in a letter to lawmakers Friday that new emails had surfaced that were related to Clinton's case, despite an FBI tradition of avoiding controversial actions in the days leading up to an election.
In a letter to FBI employees later Friday, Comey said, "Of course we don't ordinarily tell Congress about ongoing investigations," but he felt the need to because he had testified repeatedly about the investigation. "I also think it would be misleading to the American people were we not to supplement the record."
He said in trying to strike that balance, and considering "we don't know the significance" of the newly found emails, "there is significant risk of being misunderstood" by releasing a letter "in the middle of an election season."
WATCH: Democrat Clinton: Timing of FBI email probe 'strange'
Speaking to a friendly audience Saturday, Clinton said, "My mother taught me to never, ever quit." Later in her speech, she added, "We can't let this election, in the last 10 days, be about the noise and the distractions. It's got to be about what kind of country we want for ourselves, our children and our grandchildren."
Donna Brazile, the chairwoman of the Democratic National Committee and a close Clinton ally, told The New York Times on Saturday, “This is like an 18-wheeler smacking into us, and it just becomes a huge distraction at the worst possible time.”
Earlier Saturday, Republican presidential candidate Donald Trump called the latest wrinkle in the Clinton case "the lowest point in the history of our country."
Trump spoke at a campaign event in the swing state of Colorado, telling supporters, "A vote for Hillary is a vote to surrender our government." He vowed that on Election Day, November 8, "we're going to change things."
Comey's announcement that authorities were again looking into Clinton's email practices sparked criticism that his agency was intervening in the U.S. presidential race.
WATCH: Republican Trump Calls Democrat Clinton's Email Probe Bigger 'Than Watergate'
Reports Friday revealed that FBI agents discovered the new emails during an investigation into former New York Congressman Anthony Weiner, who is married to top Clinton aide Huma Abedin. In a letter to Congress, the FBI director did not disclose how many emails were discovered or whether they contained classified information.
The Washington Post reported Saturday that U.S. Justice Department officials warned Comey that notifying Congress about renewing the investigation was not consistent with department practices. The newspaper reported that authorities told Comey that the department does not comment on ongoing investigations and it does not take steps that can be seen as influencing an election.
Four senior Democratic senators urged the Justice Department and the FBI on Saturday to promptly release more information on emails identified as pertinent to its investigation of Clinton.
In a letter, the senators said the letter, released less than two weeks before the election, is being used for political purposes and has led to a misleading impression about the FBI's actions and intentions.
Senators Benjamin Cardin of Maryland, Thomas Carper of Delaware, Patrick Leahy of Vermont and Dianne Feinstein of California have asked for more information, including the number of emails involved and what steps are being taken to determine the number of emails that might have already been viewed by the FBI, by Monday.
WATCH: Clinton responds to FBI email investigation
Political historian Allan Lichtman said Saturday that Comey might have made the announcement to protect his own credibility, but that the results of his act reach far beyond the personal.
"The tragedy here is, to save himself [from allegations that he hid new evidence], he has significantly biased a presidential election that is vastly more important," Lichtman said. "This is going to set the course for the future of the country, not just over the next four years, but perhaps over the next generation. ... He has no business doing this."
Lichtman, who is known for his accuracy in predicting U.S. election outcomes, would not speculate on whether the development would sway Clinton voters. He said the Clinton campaign has probably done the only thing it could do in response, which is to call for the release of all information about the case.
"How do you respond to something that's so vague?" Lichtman said. "It may be that there's nothing there of any consequence, but we don't know and you can't draw any conclusions from the FBI letter."
Clinton made the call Friday for the FBI to disclose what it had found. "We are calling on the FBI to release all the information that it has. Even Director Comey noted that this new information may not be significant — so let's get it out."
Weiner is being investigated by federal authorities for "sexting" with an underage girl in North Carolina. He and Abedin are now separated.
Government sources said Friday that the FBI discovered new emails from Clinton's private email server after it seized devices belonging to Abedin and Weiner.
Clinton campaign manager John Podesta challenged Comey's "extraordinary" statement, which came in a letter written to senior legislators. Podesta said Comey "should immediately provide the American public more information," and added he was confident that the reopened investigation would clear Clinton of wrongdoing.
Republican Party leaders rejoiced in what they saw as validation of their repeated complaints about Clinton's practice of handling government emails in a nonstandard fashion.
"Nobody was aware this was coming," an official at the State Department told VOA, but a deputy spokesman, Mark Toner, told reporters the government agency will "certainly" cooperate fully with the FBI's inquiry.
The FBI chief said he could not predict how long it would take to complete the new phase of the investigation.
Trump, who was in New Hampshire when the FBI made its announcement, welcomed the news and said he hoped the bureau would "right the horrible mistake" it made in July.
Trump claimed the Clinton email investigation was a scandal "bigger than Watergate," the political spying effort and subsequent cover-up that forced the resignation of the late President Richard Nixon in 1974.
On Capitol Hill, the speaker of the U.S. House of Representatives, Paul Ryan, said the FBI's action was "long overdue." In a statement, he said the Clinton investigation was "the result of her reckless use of a private email server, and her refusal to be forthcoming with federal investigators."
Senator Charles Grassley, a Republican from Iowa and chairman of the Senate Judiciary Committee, asked for a briefing from the FBI regarding the email investigation.
"Congress and the public deserve more context to properly assess what evidence the FBI has discovered and what it plans to do with it," Grassley said.
He added, however, that the revelation was "going to be a real problem for the Clintons."
Influencing the Vote
Jason Johnson, a professor of political science and communications at Morgan State University, was more critical, saying Comey was attempting to influence the election without revealing any new information.
“It is impossible to separate what this announcement is from a political act. It is clearly an act on the part of the FBI director to have some impact on the presidential campaign. There is something to be said about caution, about discretion,” he said.
Johnson also said the FBI should have examined the evidence first and made an announcement on Monday, if necessary.
"This is the equivalent of someone saying, 'I found some change in my couch, and I may be a millionaire.' ... This does nothing to advance justice," he said.
Congressman Gregory Meeks, a Democrat from New York, said the announcement by Comey is "unbelievable."
Comey should not "be the one the one that's trying to sway an election 10 days beforehand," Meek told the Associated Press.
Clinton has said she used the private server rather than a more secure government server because it was more convenient for her. Critics say she was trying to shield her communications from the Freedom of Information Act.
Although classified information turned up in email stored on her personal server, which authorities say was vulnerable to hacking, there was no evidence she shared it with unauthorized parties on purpose and tried to cover that up.
Steve Herman and Jesusemen Oni contributed to this report.