Russia Is Main Suspect in Cyber Attack on Joint Staff's Emails, US Official Says

— -- The U.S. believes Russia is the leading suspect behind a cyber attack that brought down portions of the Joint Staff’s unclassified email system for almost two weeks, according to a U.S. official.

The attack launched on the Joint Staff over the July 25th weekend "bears the hallmarks of a state actor," the official said Thursday. The official said Russia is suspected of being behind the attack, though that has not been definitively determined.

The U.S. intelligence community's worldwide threat assessment released in February listed Russia and China as "nation states with highly sophisticated cyber programs." Officials said the phishing attack was so sophisticated, only a nation-state could have been capable of launching it.

Defense officials said desktop access to the system was limited after the cyber intrusion was detected. The Joint Staff’s classified email system has not been affected.

Located in the Pentagon, the 2,500 military and civilian personnel on the Joint Staff provide support and planning for Army Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff and the president’s top military adviser.

Dempsey and the rest of the Joint Staff had access to their unclassified emails through workarounds and a mobile unclassified email system that was unaffected, said defense officials.

A senior Defense official said while more sophisticated attacks on the military’s computer networks have been blunted in the past, this cyber attack used an approach that had not been seen before. Pentagon spokesman Capt. Jeff Davis said Monday the U.S. military’s computer networks routinely come under cyber attack.