The emails sent also contained a warning to Vinny Troia and a cybercriminal group, The Dark Overlord.
Apparently, the hacker worked at the U.S. Department of Homeland Security’s Cyber Threat Detection and Analysis Group which has not been operational for two years now.
The FBI has warned American companies and agencies on the issue, and this is the first case of a hacker spamming people via FBI email server.

"The FBI is aware of a software misconfiguration that temporarily allowed an actor to leverage the Law Enforcement Enterprise Portal (LEEP) to send fake emails. LEEP is FBI IT infrastructure used to communicate with our state and local law enforcement partners," 

"No actor was able to access or compromise any data or PII [personally identifiable information] on FBI’s network," 

The FBI emailed statement said.