“It was clearly the work of a state actor,” Defense Department officials told NBC, without clearly stating whether elements of the Russian government orchestrated the attack, or whether it was carried out by individuals. The hackers reportedly conducted the cyberattack using encrypted accounts on social media.

Officials told The Daily Beast the attack involved “new and unseen approaches into the network.”

No classified information was accessed or stolen, and only unclassified accounts were involved, the officials say. The Pentagon chose to shut down the system pending an investigation. It will reportedly be back online later this week.

The reported attack comes roughly two months after hackers believed to be associated with the Chinese government successfully hacked the U.S. Office of Personnel Management, stealing personal information about roughly 21 million people, including as many as four million federal employees with security clearances.