Hackers broke into US government computers, possibly compromising the personal data of 4 million current and former federal employees, with investigators probing whether the culprits were based in China, US officials said.
In the latest in a string of intrusions into US agencies' high-tech systems, the US Office of Personnel Management suffered what appeared to be one of the largest breaches of information ever on government workers. The office handles employee records and security clearances.
Chinese Embassy spokesman in Washington Zhu Haiquan said hypothetical accusations were irresponsible. He said it was hard to track hacker activity across borders.
The FBI said it had launched a probe and aimed to bring to account those responsible.
OPM detected new malicious activity affecting its information systems in April and the Department of Homeland Security said it concluded at the beginning of May that the agency's data had been compromised with about 4 million workers likely affected.
The agencies involved did not specify exactly what kind of information was accessed.
The breach hit OPM's IT systems and its data stored at the Department of the Interior's data centre, a shared service center for federal agencies, a DHS official said on condition of anonymity. The official would not comment on whether other agencies' data had been affected.
OPM had previously been the victim of another cyber attack, as have various federal government computer systems at the State Department, the US Postal Service and the White House.
Chinese hackers were blamed for penetrating OPM's computer networks last year, when hackers appeared to have targeted files on tens of thousands of employees who had applied for top-secret security clearances, the New York Times reported in July, citing unnamed US officials.
"The FBI is working with our inter-agency partners to investigate this matter," the bureau said in a statement. "We take all potential threats to public and private sector systems seriously, and will continue to investigate and hold accountable those who pose a threat in cyberspace."
The US government has long raised concerns about cyber spying and theft emanating from China and has urged Beijing to do more to curb the problem.
There was no comment from the White House.
Since the intrusion, OPM said it had implemented additional security precautions for its networks. It said it would notify the 4 million employees and offer credit monitoring and identity theft services to those affected.