Hackers responsible for the SolarWinds cyberattack aimed at a number of federal agencies and companies were able to access the email accounts of Justice Department employees.

The DOJ said on January 6 that about 3% of the departments’ Microsoft Office 365 email accounts were compromised in the hack, which analysts have cited as one of the most serious cyberattacks in recent history.

A DOJ spokesperson said that the department learned of “previously unknown malicious activity” on Dec. 24, and was able to fix the technical flaw that allowed hackers to infiltrate email accounts. The spokesperson added that the DOJ has “no indication that any classified systems were impacted.”

“The Department will continue to notify the appropriate federal agencies, Congress, and the public as warranted,” the spokesperson said.

On January 5, a federal cybersecurity task force that includes representatives of the Federal Bureau of Investigation and National Security Agency said that Russia was likely responsible for “most or all of the recently discovered, ongoing cyber compromises of both government and nongovernmental networks.”

Other federal agencies that have reportedly been impacted include the Treasury Department, Commerce Department, and Homeland Security.