The theft by a criminal hacking group of reams of sensitive personal data involving potentially tens of thousands of former and current F.B.I. employees is emerging as one of the worst breaches of sensitive government information, leaving the bureau rushing to protect its personnel as an uncertain deadline loomed. Nearly a week after the group, known as Shiny Hunters, revealed it had pilfered intimate details about bureau personnel from the agency’s jobs portal and threatened to leak them online, F.B.I. investigators are still piecing together how the breach took place and the total damage.

The hack appears to have swept up home addresses, Social Security numbers, secretive job assignments and much more, according to a New York Times analysis of some of the records. Some are already comparing it to China’s breach of more than 20 million records from the Office of Personnel Management over a decade ago, considered so catastrophic that officials and lawmakers vowed to never let something like it happen again.

In a statement on Monday, the agency said it was treating the matter with the utmost seriousness. It added, “The F.B.I. treats the security of its information and the safety of its work force as top priorities, and our investigation is ongoing.” In announcing its hack, Shiny Hunters said it had targeted the F.B.I. as retribution for a public advisory the bureau had issued in the spring warning that the group was known to harass victims and family members with threatening or coercive maneuvers.