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.

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