Television broadcasts and mobile phones in Hawaii were interrupted by an emergency warning of an incoming missile on Saturday.
The emergency sent to mobile phones warned, in capital letters: "Ballistic missile threat inbound to Hawaii. Seek immediate shelter. This is not a drill."
The message was accidentally sent at 8.07am local time (6.07pm GMT).
According to Hawaii Emergency Management Agency spokesman Richard Repoza, the alert was sent in error.
Mr Repoza said the agency was trying to determine what happened.
The US military's pacific command said that there was no ballistic missile threat to Hawaii.
According to social media reports, the alert sent many of the the islanders into a panic.
More from World
Hawaii had been named as a potential target of an intercontinental ballistic missile launched by North Korea, which has threatened the US with a nuclear strike.