According to the Bureau of Industry and Security (BIS), the branch of the Commerce Department that controls exports of commodities and technology, Commerce Department user accounts were compromised by hackers with links to Chinese ISPs.

"Through established security procedures, BIS discovered a targeted effort to gain access to BIS user accounts," said Commerce Department spokesperson Richard Mills. "We have no evidence that BIS data has been lost or compromised."

However, the attacks did force the department to take hundreds of computers offline. The department indicated it will continue to monitor the network .

Continued Breaches

The breach marked the second time in recent months that U.S. officials confirmed a major hack attack had been traced to China. In July, hackers from China broke into State Department computers.

Joe Wilcox, a Jupiter Research analyst, said protecting networks that support large numbers of users can be very difficult, especially when many of those users are mobile and access the central network from a myriad of locations.

But, he explained, in a worst-case scenario, you can still do a lot to protect sensitive data. "If an organization is doing its job correctly and security is in place, it can limit what damage is done," he said.

However, Wilcox did say that if the attack was, in fact, a rootkit installed on BIS computers -- as has been reported -- it could cause serious problems. "A rootkit is one of the most difficult types of security breaches to detect," he said.

Rootkits are essentially Trojans that have become increasingly popular among hackers over the past year because of the ease with which the secret software can bury itself at low levels of the operating system and remain hidden from antivirus applications.

Increasing Worries over China

As a result of the attacks, U.S. officials have become increasingly concerned over the Chinese gaining access to sensitive information. Earlier this year, for example, after a reprimand from Congress, the State Department abandoned plans to roll out Lenovo computers for its workers. Congress said it was concerned the Chinese manufacturer might install listening devices on the machines.

Perhaps the most well-known instance of information piracy came in 2000 when hackers -- who were suspected of working for China -- penetrated a top-secret computer system at Los Alamos National Laboratory and gleaned massive amounts of sensitive information.

Still, Wilcox gave the Commerce Department credit for discovering the attack. "It is very interesting that the department was able to detect it," he said.