The FBI will re-open its investigation into Hillary Clinton's use of a private email server, 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.

 FBI to re-open Clinton email investigation 5:56

In September, Comey defended 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 makes clear that new evidence has 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.

Asked to respond, a top Clinton campaign spokesperson tells NBC News, "No idea."

The news come less than two weeks before the general election with Clinton leading in the polls.

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