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By Jim Miklaszewski

The Joint Chiefs of Staff's unclassified email system is back online, defense officials tell NBC News, more than two weeks after it was the target of a cyberattack believed to have been carried out by Russian hackers.

Pentagon officials told NBC News the Joint Chiefs email system was restored Sunday, following an intensive "scrub" meant to eliminate any potential malware that may have been implanted. Additional security measures were also installed, the officials said. The system was originally scheduled to be back online Friday.

Related: Russian Cyber Attack Targets Pentagon Email Systems: Officials

The "highly sophisticated" cyberattack is believed to have occurred sometime around July 25 and affected about 4,000 military and civilian personnel who work for the Joint Chiefs, officials had told NBC News last week. The officials insist no classified information was compromised or stolen during the attack on the unclassified email system.

Officials said it appears the intrusion was the result of what's known as "spear phishing" — emails that look legitimate but are loaded with links that download malicious software. At least one Pentagon or military user violated protocols and security requirements by clicking into an unknown email source, the officials said.

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Despite their firm belief that "Russians" carried out the attack, defense officials still cannot confirm whether the cyberattack was sanctioned by the Russian government or carried out by independent hackers.