The FBI revealed Friday it was reviewing a new batch of emails that "appear to be pertinent" to its previous investigation into Hillary Clinton's use of a private server while she was secretary of state.



"In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation," FBI director James Comey said in a letter to the Senate Judiciary Committee. "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, as well as to assess their importance to our investigation."

A senior law enforcement official told NBC News Friday that the Comey letter was sent to the Hill "out of an abundance of caution" and to be extra-thorough.

The official said the emails were discovered "on another device."

There's no indication, the official said, that Clinton, her campaign or the State Department was withholding information. But the emails were not held by someone who was investigated in the Clinton email case, the official said.

The politically explosive revelation, which came just 11 days before Election Day, appeared to catch the Clinton campaign by surprise.

"No idea," a top Clinton campaign spokesperson told NBC News.

Tim Kaine, Clinton's vice presidential running mate, who was at an early voting site in Tallahassee on Friday told reporters "gotta read a little more, gotta read a little more" before continuing on.

The news comes as Clinton has been leading Donald Trump in the polls.

Related: Read the Letter

GOP nominee Donald Trump, at a campaign event in New Hampshire, praised the FBI for having "the courage to right the horrible mistake they made," an apparent reference to the agency's decision earlier this year to not press criminal charges against Clinton.

"I need to open with a very critical breaking news announcement," Trump said to cheers that at times drowned out his remarks. "The FBI has just sent a letter to Congress informing them that they have discovered new emails pertaining to the former Secretary of State Hillary Clinton's investigation."

"This was a grave miscarriage of justice," he said, "it is about to be corrected."

The Republican-led House Judiciary Committee also applauded the FBI's move.

"The FBI's decision to reopen its investigation into Secretary Clinton reinforces what the House Judiciary Committee has been saying for months: the more we learn about Secretary Clinton's use of a private email server, the clearer it becomes that she and her associates committed wrongdoing and jeopardized national security," the Judiciary Committee said in a statement.

Related: FBI Releases Notes From Interview With Hillary Clinton Over Emails

But Comey, who is also a Republican and who took a lot of heat in July for his agency's decision to not charge Clinton with a crime, did not say in the letter that he was reexamining the FBI's previous finding, which concluded that her handling of the emails was sloppy but not criminal.

Documents related to the case were released just before the Labor Day weekend and included a summary of Clinton's July interview with the FBI about her private email server, as well as a detailed investigative summary of the case.

The FBI language in the letter to Congress made it clear that new evidence had been discovered and thus will be reviewed — meaning FBI agents will read these emails. It is unusual for the FBI to tell Congress it is looking over newly discovered evidence in a criminal inquiry that was otherwise closed.

Federal practice is not to comment on ongoing investigations, or discuss details of concluded investigations. Comey previously explained his departure from that practice in his earlier congressional testimony, given the special nature of this case and congressional oversight inquiries.

Still, some congressional Democrats told NBC News Friday that they are frustrated that Comey issued the letter reopening this issue without providing much in the way of specifics.

This is a developing story. Please refresh for details