An unknown user or users logged into the email account of someone connected to Hillary Clinton’s private server and accessed email folders and attachments, according to FBI documents released Friday.

An FBI document states forensic analysis found that the email of one of former President Bill Clinton’s staffers was compromised on January, 5, 2013, while Hillary was still serving as secretary of state. “Three IP addresses matching known Tor exit nodes were observed accessing a user e-mail account on the Pagliano Server believed to belong to President Clinton’s staffer [redacted],” the FBI document states. “FBI investigation indicated the Tor user logged in to [redacted] e-mail account and browsed e-mail folders and attachments.”

Tor is a free software that allows users to conceal their identify through routing their traffic through servers located around the world. The document went on to say, “FBI investigation to date was unable to identify the actor(s) responsible for this login or how [redacted] login credentials were compromised.”

The FBI documents state that several attempts were made to hack into Clinton’s private server through brute force, which means the guessing of login credentials. The FBI states in the documents their investigation “did not find evidence confirming that Clinton’s e-mail server systems were compromised by cyber means.” Though a document states, “the FBI’s inability was unable to recover all server equipment and the lack of complete sever log data for the relevant time period limited the FBI’s forensic analysis of the sever systems.”