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However, at the time of the attack it was reported that no classified information relating to military operations was lost.

The incident took place in June of last year, but is still under investigation by the government. The attackers managed to insert malicious code into various computer systems, which remained undetected for two months.

Restoring the affected systems caused three weeks of downtime and incurred costs of $4 million.

"This was a very bad day," said Dennis Clem, the chief information officer at the Office of the Secretary of Defense, speaking at the Information Processing Interagency Conference this week.

"They're working hard, these people, and they're after us all the time... If you don't know your network, and you're more of a policy CIO, you may find yourself in trouble," he added.