In 2013, an unknown user accessed an email account on Hillary Clinton’s private email server through Tor, the anonymous web surfing tool, according to new FBI documents.

On Friday, the FBI provided details on the possible breach in newly released files about its investigation of Clinton’s use of a private email server when she was the U.S. secretary of state.

The affected email account belonged to a member of Bill Clinton's staff. In January 2013, an unknown user managed to log in to the account and browse email folders and attachments.

The FBI later interviewed the staffer, who said she had never used Tor. The tool is popular among hackers, journalists and activists to help mask their online presence.

The agency’s investigation so far hasn’t found the actor responsible or how the login credentials were obtained.

The FBI has said Clinton was “extremely careless” in her use of the server, but in a July report, the agency didn’t recommend bringing charges against her.

The new documents released on Friday said the FBI found no evidence confirming that Clinton’s email server system was ever compromised. Still, the agency said that the server had faced ongoing threats from possible hackers, including phishing email attacks and failed login attempts.

Bryan Pagliano, a Clinton aide who helped administer the server, was interviewed in the FBI’s investigation. Although Pagliano said there were no security breaches, there were many failed login attempts, or “brute force attacks,” according to the FBI documents.

At one point, “Pagliano recalled finding ‘a virus,’ but could provide no additional details, other than it was nothing of great concern,” the FBI said. The agency also found “multiple occurrences” of phishing attacks against Clinton’s email account.

In July, FBI director James Comey said it’s impossible to rule out that Clinton’s server could have been hacked.