An incoming missile alert plunged residents of Hawaii into panic on Saturday morning before it was declared a false alarm.
Mobile phone users received a message saying: "Ballistic missile threat inbound to Hawaii. Seek immediate shelter.This is not a drill."
State Governor David Ige apologised and said it was caused by an employee pressing the wrong button.
The US government announced there would be a full investigation.
An alert system is in place because of the potential proximity of Hawaii to North Korean missiles.
In December, the state tested its nuclear warning siren for the first time since the end of the Cold War.
The false warning message was sent to people's mobile devices, and was also broadcast on television and radio stations.
The phone message notification, all in capital letters, went out at 08:07 (18:07 GMT).
It was corrected by email 18 minutes later but there was no follow-up mobile text for 38 minutes, the Honolulu Star-Advertiser reports.
Television and radio broadcasts across the state were interrupted with a recorded emergency message instructing people to stay indoors.
"If you are outdoors seek immediate shelter in a building. Remain indoors well away from windows. If you are driving pull safely to the side of the road and seek shelter in a building while laying on the floor. We'll announce when the threat has ended. This is not a drill!"
People in the US state have been sharing stories of momentary frenzy and the panic-stricken messages they exchanged with loved ones after they received the alert, reports BBC.