Investigations into a hacking attack on the email system for the Joint Chiefs of Staff has revealed that Russia is behind the cyberassault that shut down their system for 11 days.

The hackers broke into an unclassified email network using malware or “phishing” attempts, meaning an email recipient had to open an infected attachment to an e-mail to allow the malicious programs to access the system.

An official with the joint chiefs called the attack the “most sophisticated” attack on their network.

Another official told CBS News that the attack impacted the 4,000 personnel who work for the Chairman of the Joint Chiefs.  Most of those personnel are military members.

Pentagon officials repeatedly stated that the classified email network was not impacted and said the Joint Chiefs were given an alternative method to send unclassified emails while the system was shut down to clear out the malicious software.

The attack is the latest in several high-profile attacks on the U.S. Government’s email systems this year.  Previous attacks were attributed to Chinese hackers.