U.S. law enforcement officials say FBI investigators knew weeks ago that emails found in a separate probe may be related to the Hillary Clinton email case, but did not reveal their discovery until Friday - just 11 days before the presidential election.
It is unclear why FBI Director James Comey waited until just before the election to announce the probe or when he knew about it.
The Clinton campaign and Democrats were delighted when Comey said in July that the FBI's investigation into Clinton's "sloppy" handling of emails when she was secretary of state would be closed with no criminal charges.
But the possibility that the probe could be reopened has got Democrats perplexed.
“It’s pretty strange to put something like that out, with such little information, right before an election,” Clinton said.
“It’s just extremely puzzling,” said vice presidential nominee Tim Kaine on ABC’s This Week program. “I just have no way of understanding these actions. They are completely unprecedented.”
Senate Democratic Leader Harry Reid went one step further Sunday by saying that Comey might have violated a law that prohibits federal employees or officials from using their positions to influence an election. He said the information released about the new emails was neither conclusive nor pertinent. "Through your partisan actions, you may have broken the law," Reid said in a letter to Comey.
Trump seizes moment
Republican nominee Donald Trump jumped right on the latest FBI revelation.
“A vote for Hillary is a vote to surrender our government to public corruption, graft, cronyism that threatens the survival of our Constitution itself,” he said.
“There’s this constant cloud of corruption that follows Hillary Clinton around,” said Trump’s campaign manager, Kellyanne Conway, also on This Week. “And for the FBI to make this remarkable move 11 days before the election means there must be something there.”
The latest emails were uncovered in a separate FBI investigation of former Congressman Anthony Weiner, the estranged husband of top Clinton aide Huma Abedin.
Weiner is accused of exchanging sexually explicit emails with a 15-year-old girl. A separate batch of emails that the FBI believes may be related to the Clinton probe were found on a computer allegedly shared by Weiner and Abedin.
Comey himself said in his notification to Congress that there may not be any significance to the newly found Clinton emails, and that they still have not been thoroughly reviewed.
It is also unclear if Clinton herself wrote or received the mails, only that the FBI believes they are germane to the case.
Only on Sunday did federal investigators obtain a warrant to begin searching the new cache of emails, law enforcement officials said.
“It happens close to an election, which is in violation of normal Justice Department protocol, and it involves talking about an ongoing investigation, which also violates the protocol,” Kaine said.
In an interview with CBS's Face the Nation taped before the Comey announcement and broadcast Sunday, Vice President Joe Biden said Hillary Clinton faces what he calls a "double standard" when it comes to trustworthiness.
"Hillary said herself ...'Look, I'm not that good a candidate,'...and a lot of it has to do with personal style. She is more measured and she makes fewer mistakes than I make or most people I know. It doesn't go to her integrity or honesty. It goes to her style."
Millions of Americans already have cast early ballots in states across the country. What effect the FBI announcement might have on turnout is unclear, but polls showed a tightening race between Clinton and Trump even before Friday’s bombshell.
VOA's Ken Schwartz contributed to this report.