An October surprise exploded in the presidential race Friday.

The FBI suddenly announced that it is reopening its investigation into Hillary Clinton’s emails after learning of new documents “that appear to be pertinent to the investigation.”

The bureau’s stunning move came less than two weeks before Election Day.

“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,” FBI Director James Comey said in a letter to House Oversight Committee Chairman Jason Chaffetz.

Comey said it is unknown whether “this material may be significant.”

He gave no timeline for when results of their new investigation might be known.

“Case reopened,” tweeted Chaffetz.

Just last month, the FBI director rebuffed requests from Republicans to reopen the investigation, saying he hadn’t seen new evidence that “would come near” taking such an extraordinary step.

Donald Trump’s campaign cheered the FBI’s decision.

“A great day in our campaign just got even better. FBI reviewing new emails in Clinton probe,” campaign manager Kellyanne Conway said in a tweet.

House Speaker Paul Ryan said, “Yet again, Hillary Clinton has nobody but herself to blame.”

“She was entrusted with some of our nation’s most important secrets, and she betrayed that trust by carelessly mishandling highly classified information. This decision, long overdue, is the result of her reckless use of a private email server, and her refusal to be forthcoming with federal investigators.”

Another top Republican said it was becoming clearer that Clinton “jeopardized national security.”

“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,” Rep. Bob Goodlatte (R-Va.), chairman of the House Judiciary Committee, said in a statement.

“Now that the FBI has reopened the matter, it must conduct the investigation with impartiality and thoroughness. The American people deserve no less and no one should be above the law.”