Russian hackers successfully infiltrated computers at the Pentagon as recent as two weeks ago, NBC News reports. Officials say that the cyberattack was "sophisticated" and hit an unclassified email server this is used by the Pentagon's Joint Staff. The email system has been shut down for around two weeks while an investigation is still taking place.

The officials could not pinpoint an exact time of when the hack took place, but told NBC News it occurred "around July 25." Along with having trouble figuring out when the cyberattack occurred, officials told NBC News they don't yet know if the Russia government is to blame or if individuals worked on their own. But, one person told NBC News that "It was clearly the work of a state actor," because of just how sophisticated the hack was.

The attack affected 4,000 military and civilian personnel. The report assures that "no classified information was seized or compromised." However, when officials first recognized the hack on the United States Office of Personnel Management in June, they said it was not severe. In July the office confirmed that 22 million Americans were affected in the cyberattack.