After reporting its unclassified email servers where taken offline for maintenance, the U.S.  Department of State has confirmed the effort was in response to a potential security breach.

The department's classified email system is believed to have remained secure. The unclassified system is expected to be back up and running soon, after receiving security enhancements.

A government official, speaking on the condition of anonymity, confirmed the federal officials' suspicions that the unclassified email network had been hacked. The official said the system was taken offline on Nov. 14 for maintenance, though the intrusion is believed to have taken place in late October.

The agency is expected to officially confirm the intrusion when it brings the shuttered servers back online today or tomorrow.

The hacking incident comes roughly a week after the U.S. Postal Service reported several of its internal systems had been compromised by hackers and weeks after hackers breached the White House network.

The unnamed government source wouldn't speculate on the identities of the individuals behind the attacks, though the perpetrators are believed to be the same group that launched the attack against the White House's network.

Hackers backed by the Russian government are believed to have launched the attack against the White House's computer systems. The White House won't point fingers, only stating that a "variety of actors" find its networks to be attractive targets.

"This is consistent with espionage activity," said Stephen Ward, senior director at software security firm iSight. "All indicators from a targeting and lures perspective would indicate espionage with Russian national interests."

A group supported by the Chinese government is suspected of perpetrating the breach against the postal service call centers and employee databases. The attack exposed the personal details of approximately 800,000 of the organization's current and former employees. Security experts believe the breach was state-funded as the hackers bypassed financial details and targeted information that could be used for counterintelligence.

"For the Chinese, this is probably a way of building their inventory on U.S. persons for counterintelligence and recruitment purpose," said James A. Lewis, an expert on cyber policy at the Center for Strategic and International Studies.

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