The FBI is reviewing a new batch of Hillary Clinton emails, bureau director James Comey said in a letter to the Senate Judiciary Committee on Friday.
"In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation … 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," he wrote.
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FBI to re-open Clinton email investigation 5:56
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.
But the Republican-led House Judiciary Committee quickly 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," their statement read.
Comey, who is also a Republican, took a lot of heat in September for his agency's decision to not press criminal charges in the investigation into Clinton's use of a private email sever.
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.
The news comes as Clinton has been leading Donald Trump in the polls.
Trump, at an event in New Hampshire on Friday, responded to breaking news by praising the FBI for having "the courage to right the horrible mistake they made" earlier in the summer.
"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."
"I have great respect for the fact that the FBI and DOJ are now willing to have the courage to right the horrible mistake they made," said Trump, who had recently been sounding the alarm on the campaign trail to "investigate the investigation" into Clinton's server.
"This was a grave miscarriage of justice," the Republican nominee said, relishing the best news he got just minutes before taking the stage in New Hampshire. Of that "miscarriage," Trumps said he hopes "it is about to be corrected."