August 7, 2015 9:00 AM EDT

U.S. officials believe Russia may be behind a cyber attack on the Pentagon that caused the Joint Chiefs of Staff to take their email offline two weeks ago.

Unnamed sources shared their suspicion with NBC News, but said it is not confirmed, and that they don’t know whether the Russian government or individuals were responsible. An official told ABC News that it “bears the hallmarks of a state actor.”

Officials say they detected the hack around July 25 and took the unclassified email system offline then; the classified system was reportedly not affected. The attack reportedly affected about 4,000 employees.


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