He contended that she threatens United States security and cannot be trusted in the White House.
"I have great respect for the fact that the FBI and Department of Justice are now willing to have the courage to right the horrible mistake that they made," Trump said at a rally in New Hampshire. "This was a grave miscarriage of justice that the American people fully understood and is about to be corrected."
"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, as well as to assess their importance to our investigation," Comey wrote.
"Although the FBI cannot yet assess whether or not this material may be significant, and I cannot predict how long it will take us to complete this additional work, I believe it is important to update your Committees about our efforts in light of my previous testimony," he concluded.
Trump claimed that "Clinton's corruption is on a scale we have never seen before."
A representative for the Clinton campaign told NBC News that the team had "no idea" what Comey was referencing in his Friday letter. The State Department declined to comment to Reuters.
Comey previously announced that his department did not recommend that the Justice Department bring charges against Clinton for her handling of classified information on a private server. In that July speech, Comey had insisted that the decision was made without outside influence and that it was the professional judgment of the FBI which led to his announcement.
— CNBC's Everett Rosenfeld and AJ Vielma contributed to this report.