An incoming missile alert plunged residents of Hawaii into panic on Saturday morning before it was declared to be false.
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 to Hawaiians, saying an employee had pressed 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.