Hackers have accessed an FBI system and sent tens of thousands of fake emails warning of a possible cyber attack.
The messages appeared to come from an FBI email address ending in @ic.fbi.gov, the FBI said.
The hardware affected in the incident was taken offline soon after the issue was discovered. "This is an ongoing situation," said the FBI.
The hackers sent tens of thousands of emails warning of a possible cyber attack, threat-tracking organisation Spamhaus Project said on its Twitter account.
A copy of an email posted by Spamhaus on Twitter showed a subject line of "Urgent: Threat actor in systems" and appeared to end with a sign-off from the Department of Homeland Security.
Both the FBI and Cybersecurity and Infrastructure Security Agency are aware of the incident, authorities said.