Hackers have broken into a network housing sensitive information on federal employees, including a database of people with top-secret clearances.
The attack occurred in mid-March, U.S. government officials told CNNMoney on Thursday. A New York Times report attributed the operation to China -- although it was unclear whether hackers were independent or government spies.
President Barack Obama's cybersecurity advisers would not say whether the attack came from China or elsewhere.
The hackers managed to break into the computer network at the Office of Personnel Management, which stores data about federal employees. The agency conducts background checks for anyone working with the federal government, and it keeps information about employee hiring, wages, pensions and security clearances.
All of this is valuable information for foreign governments and corporations. If you know who has top-secret access, you know who to bribe -- or kidnap -- to get information.
The hackers managed to break into the agency's network, according to several federal agencies involved. But U.S. investigators haven't yet spotted "any loss of personally identifiable information," according to an official at the Department of Homeland Security who spoke to CNNMoney on condition of anonymity.
DHS and the personnel agency constantly monitor computer systems, so the break-in set off alarms and allowed them to act quickly, according to Office of Personnel Management press secretary Nathaly Arriola.
The U.S. Computer Emergency Readiness Team is now conducting forensic analysis to determine how deep the hackers got -- and what they know.
This news comes at a pivotal moment: just U.S. Secretary of State John Kerry wraps up the United States' annual in-person meeting with Chinese officials. On Thursday, America's top diplomat released a joint statement with Chinese State Councilor Yang Jiechi discussing cybersecurity -- but not this latest episode.
They called cybersecurity "a common threat" and said China hopes to "strengthen cooperation on the basis of mutual respect and trust."
However, today's revelations put that in context. The reality is, there's an ongoing cyberwar. On one side, there are the all-seeing American and British cyber spies. On the other are intellectual-property-stealing hackers in China and Russia.
And there's no end in sight.