Hackers have targeted a Federal Bureau of Investigation (FBI) email system by sending thousands of messages warning of a cyber attack.
Fake emails appeared to come from a legitimate FBI email address ending in @ic.fbi.gov, the agency said in a statement.
A copy of one of the fake emails sent was posted on Twitter by the threat-tracking organisation Spamhaus Project.
It included a subject line that read "Urgent: Threat Actor in systems" and appeared to end with a sign-off from the Department of Homeland Security.
It also said: "Our intelligence monitoring indicates exfiltration of several of your visualised clusters in a sophisticated chain attack."
It continued to read that "Vinny Troia, whom is believed to be affiliated with the extortion gang TheDark0verlord", had been identified as the "threat actor", before recommending people to check their systems.
Spamhaus Project said the hackers sent thousands of emails warning of a possible cyber attack.
The FBI said the hardware impacted by the incident "was taken offline quickly upon discovery of the issue".
"This is an ongoing situation," it added in a statement.
The hack was first reported by Bloomberg, which, citing Spamhaus, said the attacks started at midnight on Saturday in New York with a subsequent campaign beginning at 2am.