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Pentagon computers back online: NBC, citing sources

The Pentagon building in Washington, DC.
AFP | Getty Images
The Pentagon building in Washington, DC.

Defense officials told NBC News the Joint Chiefs of Staff unclassified email system went back online, more than two weeks after it was the target of a highly sophisticated cyberattack believed to have been carried out by Russian hackers.

Pentagon officials said the service was restored Sunday following an intensive scrub of the system to eliminate any potential malware that may have been implanted during the intrusion. Additional security measures were also installed.

Read MoreRussia hacks Pentagon computers

The officials said it appears the intrusion was the result of spear phishing when one or more Pentagon or military users had violated protocols and security requirements by clicking onto an unknown email source. The officials insisted that no classified information was compromised or stolen during the attack on the unclassified email system.

Despite the firm belief that Russians carried out the attack, DOD officials still could not say whether the cyberattack was sanctioned by the Russian government or carried out by independent hackers.