The fake emails were sent from a legitimate FBI address ending in @ic.fbi.gov, according to the agency's statement.

It is unclear how much access the hacker or hackers gained to the FBI's system, however the email which was spammed was a bizarre screed which references Vinny Troia - CEO of Night Lion Security, which published research on The Dark Overlord hacking group in January.

According to NBC News, the hacker signed off as the US Department of Homeland Security's Cyber Threat Detection and Analysis Group - which has been defunct for at least two years.

The incident comes on the heels of a number of high-profile breaches of U.S. government networks in recent months, including a Russia-based attack that compromised at least nine federal agencies, and a Chinese-based hacking campaign so severe that the Cybersecurity and Infrastructure Security Agency had to issue a rare mandate for all government agencies to immediately update their software. -NBC News

Researcher Alex Grosjean of the Spamhaus Project based in Europe said that the emails' metadata made clear that they were in fact sent from an FBI server.