According to Bleeping Computer, the FBI has confirmed intruders compromised its email servers on November 13 to send fake messages claiming recipients had fallen prone to data breaches.
The emails tried to pin the non-existent attacks on Vinny Troia, the leader of dark web security firms NightLion and Shadowbyte, reports Engadget.
The non-profit intelligence organisation Spamhaus quickly shed light on the bogus messages.
The attackers used legitimate FBI systems to conduct the attack, using email addresses scraped from a database for the American Registry for Internet Numbers (ARIN), among other sources.
Over 100,000 addresses received fake emails in at least two waves.
The FBI described the hack as an "ongoing situation" and did not initially have more details to share, the report said.
It asked the email recipients to report messages like these to the bureau's Internet Crime Complaint Center or the Cybersecurity and Infrastructure Security Agency, it added.