WASHINGTON — (CNN) A key military email system remains offline, one week after a cyberattack that one Defense Department official described as “serious.”

“What a mess,” another Defense official added.

For nearly a week, some 4,000 key military and civilian personnel working for the Joint Chiefs of Staff have lost access to their unclassified email after what is now believed to be an intrusion into the critical Pentagon server that handles that email network, several officials have told CNN.

The incident, under investigation by U.S. Cyber Command, has been expanded to look at whether the intrusion has also impacted other government networks and servers.

Officials emphasize that, so far, no classified systems have been impacted. The military took the system offline once the intrusion was detected.

The attack is being broadly described as a spear phishing attack to multiple users on the system. These attacks are generally aimed at stealing personal information.

The attack’s sophistication suggests the possibility of a “state entity,” rather than an individual being responsible, one official told CNN.

The unclassified Joint Chiefs of Staff email network serves all the staffers that work for Gen. Martin Dempsey, the chairman of the Joint Chiefs, on everything from operational to budget matters.

The most senior officials have been given alternative email systems, and much of the work has shifted for now to a highly secure classified network.

There is no estimate when the system will be brought back online, those officials said.

The incident, which has not been publicly revealed in detail, began last weekend, when “suspicious” cyber activity was detected on the Joint Staff network. The network was immediately taken offline, officials say.

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