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The Pentagon took its Joint Staff unclassified email system offline nearly two weeks ago, after detecting a "sophisticated cyberattack" by alleged Russian hackers, U.S. officials told NBC News on Thursday.

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

The officials said the suspected Russian hackers coordinated the cyber attack via social media accounts. It’s not clear whether the attack was sanctioned by the Russian government or was the work of individuals.

The officials said that no classified information was seized or compromised and that only unclassified accounts and emails were hacked.

Almost immediately after the cyber attack was detected the Pentagon shut down the entire Joint Staff unclassified email system. The system is expected to be back online before the end of this week.