Chinese hackers are suspected of breaking in to US government computers and stealing data from at least four million federal workers, officials have said.
The Department of Homeland Security (DHS) said that data from the Office of Personnel Management (OPM) had been compromised.
Cyber investigators linked the breach to earlier thefts of healthcare records from Anthem, the second largest US health insurer, and Premera Blue Cross, a healthcare service provider.
“The FBI is conducting an investigation to identify how and why this occurred,” the statement said.
US officials said the investigators were probing whether the culprits were based in China.
China responded, saying any allegations that it was involved in breaking into US government computers were irresponsible and unscientific.
Susan Collins, a member of the Senate intelligence committee, said the breach was “yet another indication of a foreign power probing successfully and focusing on what appears to be data that would identify people with security clearances”.
The OPM has suffered what appears to be one of the largest breaches of information ever on government workers, as the office handles employee records and security clearances.
It said it became aware of the breach in April during an aggressive effort to update its cyber-security systems.
OPM said it would be contacting all those individuals whose personal data may have been breached in the coming weeks and offering them 18 months of free credit monitoring and identity theft insurance.
Information stored on OPM databases includes employee job assignments, performance reviews and training.
Several US states were already investigating a cyber-attack on Anthem in February that a person familiar with the matter said is being examined for possible ties to China.
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, and 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 2014, citing unnamed US officials.