Officials told NBC that what they termed “sophisticated cyber intrusion” occurred around July 25 and affected some 4,000 military and civilian personnel who work for the Joint Chiefs. The e-mail system was shut down and has been off line for nearly two weeks.

While the officials said it’s not clear whether the attack was sanctioned by the Russian government or conducted by individuals, given the scope of the attack, “It was clearly the work of a state actor.”

According to the officials, the cyberattack relied on an automated system that rapidly gathered massive amounts of data and within a minute distributed all the information to thousands of accounts on the Internet, NBC reported. The suspected Russian hackers coordinated the cyberassault via encrypted accounts on social media, officials told NBC.

No classified information was seized or compromised, officials told NBC.

“There is a Cold War in cybersecurity and that’s a fact,” FireEye Inc. CEO Dave DeWalt said earlier this month. The cybersecurity expert who formerly led McAfee said the majority of cyber attacks are initiated by groups affiliated with a government entity.

“There’s a grand chess game being played out in cyber,” he said.

In July, The Daily Beast reported that the Defense Department warned that “at least five” DOD computer users have been targeted and linked the July attacks to the hacking of unclassified networks at the White House and State Department that began last year.