The FBI is reopening the investigation into Hillary Clinton‘s use of a private email server, the bureau’s director said in a letter to lawmakers on Friday, a surprise twist in a turbulent campaign just 11 days before election day.

In a note to congressional committee chairs, FBI director James Comey said that the FBI had discovered additional emails relevant to the investigation into Clinton’s and that agents were examining the emails to determine whether they contain classified information.

There was no additional information about what the emails contain or how long the investigation could take.

“In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation,” said Comey. “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.”

Also unknown is what the unrelated case is that led to the FBI discovering additional emails.

Clinton’s use of a private email server during her time as Secretary of State has become a central issue of the campaign.

After a months-long investigation, the FBI decided earlier this year not to recommend criminal charges against Clinton, saying there was no prosecutorial precedent to do so.

At question in the investigation was the former secretary of state’s handling of classified information. The FBI found that in a handful of instances, classified email was discovered on Clinton’s personal server.

This article was originally published on Time.com.