Russia is "strongly suspected" of being behind a sophisticated cyberattack against the Pentagon's Joint Staff email system, Defense Department officials told Fox News on Thursday.
"The incident bore the hallmark of a state actor," a Pentagon spokesperson told Fox News.
A separate official told Fox that the only two countries capable of carrying out such an attack are Russia and China. While officials say they "strongly suspect" Russia, they cannot pin the blame on it with 100 percent certainty.
The attack occurred on July 25 and affected the entire Joint Staff e-mail system, which has been offline for two weeks. Nearly 4,000 Defense Department employees and civilian personnel were affected by the cyberattack.
Officials told NBC News, which first reported the story, that no classified information had been seized or compromised.
It is at least the second time this year that Russia has been suspected of a hack against the Pentagon. In April, Defense Secretary Ash Carter blamed Russia for a DOD breach into an unclassified defense computer network earlier in the year.
-- Fox News' Lucas Tomlinson and Jennifer Griffin contributed to this report.