Hackers with suspected links to China appear to have accessed sensitive data on US intelligence and military personnel, American officials say.
The details of a major hack emerged last week, but authorities have given details of a possible second offense.
It is feared that the attack could leave the security personnel of the United States or their families open to blackmail.
The agency involved, the Office of Personnel Management (OPM), however, is to comment on the reports.
The officials, who spoke on condition of anonymity to news agency Associated Press (AP), believe that attackers have turned to forms submitted by intelligence and military personnel security clearances.
The document includes personal information – everything from eye color, financial history, past substance abuse, as well as contact details for friends and relatives of the person.