The Pentagon took the unclassified email network used by chairman of the Joint Chiefs of Staff Gen. Martin Dempsey as well as hundreds of military and civilian personnel offline over the weekend, citing suspicious activity.
According to CNN, Pentagon spokesman Lt. Col. Valerie Henderson said Tuesday that the U.S. military’s Cyber Command is currently investigating the issue, though officials would not specify details of what happened.
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Pentagon officials have said that there is no indication classified systems were compromised by the suspicious activity on the unclassified computer system.
An unnamed defense official said Dempsey and his colleagues have been given a "work around" to gain access to their unclassified email accounts as the investigation persists.
The official said the Pentagon is focusing on "mitigating the cyber security risks we see across our impacted network."
The news comes just weeks after the Office of Personnel Management admitted that a massive cyber attack originating from Chinese sources compromised the personal data of 22.1 million Americans.
According to an internal report by the Department of Homeland Security, the attack on OPM networks is believed to be part of a large-scale operation aimed to steal information for intelligence or military reasons.
The commander of U.S. Cyber Command Adm. Mike Rogers said Monday that the OPM hack is indicative of a new trend toward using Big Data analytics to carry out cyber attacks.