According to the officials, the "sophisticated cyber intrusion" occurred sometime around July 25 and affected some 4,000 military and civilian personnel who work for the Joint Chiefs of Staff.

Sources tell NBC News that it appears the cyberattack relied on some kind of automated system that rapidly gathered massive amounts of data and within a minute distributed all the information to thousands of accounts on the Internet. The officials also report the suspected Russian hackers coordinated the sophisticated cyberassault via encrypted accounts on social media.

The officials say its not clear whether the attack was sanctioned by the Russian government or conducted by individuals. But, given the scope of the attack, "It was clearly the work of a state actor," the officials say.