The FBI is not through investigating Hillary Clinton’s private email server.

Though Clinton was cleared of criminal charges by the FBI in July, Director James Comey sent a letter to members of Congress on Friday that it has come across new emails in “an unrelated case,” and that the bureau will review them “to determine whether they contain classified information, as well as to assess their importance to our investigation.”

The announcement is a major surprise that comes with less than two weeks until the November 8 presidential election, and will almost certainly be used by Donald Trump in the home stretch of the campaign.

What Comey said

Comey’s letter is short and does not contain many details. For instance, it does not make clear what new emails have been discovered, and doesn’t indicate “whether this material may be significant," how they relate to Clinton’s case, or how long it will take the FBI to conduct its investigation.

But Comey does suggest the FBI will be looking to see if Clinton or her aides broke the law by sending classified information over the private server she used exclusively while secretary of state. (See Vox’s Alvin Chang’s helpful explainer about why the government has made it illegal to knowingly send classified information over an unsecured server.)

Clinton has also been accused by some of violating open records laws by only maintaining a private server. Separately, some conservatives have now maintained that she broke the law by having private emails deleted in violation for a congressional subpoena.

Comey’s investigation does not appear to be about either of these distinct allegations. He writes (emphasis added):

In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation. I am writing to inform you that the investigative team briefed me on this yesterday, and I agreed that the FBI should take appropriate investigative steps designed to allow investigators to review these emails to determine whether they contain classified information.

What anonymous government sources are telling reporters about the new emails

We don’t yet have any public confirmation from Comey or the FBI about just what these new emails are, what they say, what they might show, or how they turned up.

But both the New York Times and CNN are reporting, from anonymous government sources, that the new emails came from devices seized during an FBI investigation into Anthony Weiner, who is married to a close Hillary Clinton aide, Huma Abedin. NBC’s Pete Williams reports that emails between Abedin and Clinton were found on Weiner’s laptop:

.@PeteWilliamsNBC—In looking at Weiner's laptop, investigators discovered Huma also used the laptop—which contained some Huma/Hillary emails pic.twitter.com/v1FKrYtrJD — Bradd Jaffy (@BraddJaffy) October 28, 2016

Weiner is reportedly under investigation for reports that he exchanged explicit sexts with an underaged girl. The Times’ Adam Goldman and Alan Rappeport write that devices belonging to both Abedin and Weiner were seized during the investigation, according to federal law enforcement officials.

Earlier, the Associated Press reported that the new emails did not come from Clinton’s private server, citing to a US official. And NBC’s Pete Williams reported during a TV segment that, according to his sources, the new emails are not from Hillary Clinton herself and they don’t appear to involve evidence that she or her people deliberately withheld from the FBI.

As to why the letter was sent and just how important the new emails are, Williams says:

“It looks at this point like being very thorough and very careful, not that this is going to be — again, we don’t know all the facts here — but not that it’s going to be a game-changer. One official said, ‘I don’t sense alarm bells going on in the FBI and the Justice Department over this, that they need to now totally rethink it.’

Williams added that “there’s absolutely no way that this is going to be resolved in 11 days,” so it’s not clear how much more information we’ll get.

The Clinton campaign called on the FBI to release more information

John Podesta, the chairman of the Clinton campaign, called the FBI’s decision to release the vague letter “extraordinary” and implied that Comey might be bowing to politcial pressure, saying Trump has been “baselessly second-guessing the FBI and, in both public and private, browbeating the career officials there to revisit their conclusion in a desperate attempt to harm Hillary Clinton's presidential campaign.”

Comey should provide more information “immediately,” Podesta said, saying that the letter revealed no information about what emails came to light or whether they were significant.

"It is extraordinary that we would see something like this just 11 days out from a presidential election,” Podesta said. "The Director owes it to the American people to immediately provide the full details of what he is now examining."

This is some good news for Donald Trump

Clinton’s emails have long been a political vulnerability for her — it’s an issue she’s had to respond to at length in interviews and debates — and so Trump’s campaign is understandably thrilled by the prospect of yet more bad news about emails. It’s also the first time in a while that a surprise bombshell has pertained to Clinton rather than Trump, who has been battling allegations of sexual assault since the secret recording of his comments about women was published three weeks ago.

Trump was at a rally in New Hampshire when the news broke, but his campaign manager Kellyanne Conway was jubilant on Twitter:

A great day in our campaign just got even better. FBI reviewing new emails in Clinton probe @CNNPolitics https://t.co/WBltG2lAK6 — Kellyanne Conway (@KellyannePolls) October 28, 2016

Trump seized on the issue immediately at his rally: ““Hillary Clinton’s corruption is on a scale we have never seen before. We must not let her take her criminal scheme into the Oval Office,” he said in a statement. “I have great respect for the fact that the FBI and the DOJ are now willing to have the courage to right the horrible mistake that they made.

Trump on Clinton's emails: "This is bigger than Watergate." — Tasneem N (@TasneemN) October 28, 2016

Speaker of the House Paul Ryan, meanwhile, said Clinton should stop receiving classified briefings, saying she has no one to blame but herself:

.@SpeakerRyan statement on FBI reopening Clinton email investigation: "Hillary Clinton has nobody but herself to blame." pic.twitter.com/2VgSBEG4l4 — This Week (@ThisWeekABC) October 28, 2016

This story has been updated with new developments.

Watch: Understanding how Hillary Clinton would govern