A prolific criminal hacking group said Tuesday that it had stolen a large tranche of sensitive personnel information belonging to thousands of F.B.I. officials in what would amount to a stunning breach of private data for the nation’s top law enforcement agency. The hackers, who call themselves Shiny Hunters, said the records were stolen from an online jobs portal for the bureau and included names of current and former agents as well as applicants and corresponding home addresses, phone numbers, names of spouses, certain medical information and other data.

The group does not appear to have yet leaked any of the data publicly. “We have compromised the FBI,” the group said in a message posted online that was addressed to the F.B.I. director, Kash Patel, and Brett Leatherman, who oversees the bureau’s cybersecurity division. “We hold very sensitive data on almost ALL FBI Agents and individuals who filed an application with the FBI for a job.”

In its message, Shiny Hunters said it had targeted the F.B.I. as retribution for a public advisory the bureau issued in the spring warning about the group’s tactics. The group demanded the F.B.I. “correct or simply REMOVE” the advisory within a week or risk further consequences. An F.B.I. spokeswoman acknowledged the breach, saying that the bureau was aware of the claims and that it was investigating. The jobs website remained inaccessible Wednesday morning after its homepage a day earlier featured a banner that read, “THIS SITE HAS BEEN SEIZED BY SHINYHUNTERS.”

Current and former F.B.I. officials and cybersecurity experts who track criminal groups said that the breach appeared authentic and that it amounted to a potentially devastating security lapse and counterintelligence danger for the bureau. Among other concerns, the personnel information of F.B.I. officials, if posted on the dark web, could be acquired by foreign intelligence agencies in China, Russia or elsewhere and weaponized for espionage operations.