An incoming missile alert plunged residents of Hawaii in the US into panic before it was declared a false alarm.
Mobile phone users on Saturday 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, BBC reported.
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.
It was corrected by email but there was no follow-up mobile text for 38 minutes, the Honolulu Star-Advertiser reports.
Governor Ige said human error during one of the thrice-daily shift changes at the the state's Emergency Management Agency (EMA) was to blame for the false alert.
"It was a procedure that occurs at the change of shift where they go through to make sure that the system, that it's working. And an employee pushed the wrong button," he explained.
"It was an inadvertent mistake," said EMA administrator Vern Miyagi. "The change of shift is about three people. That should have been caught... it should not have happened."
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.
Videos posted on social media appeared to show students at the University of Hawaii running for shelter after the missile threat was issued.
Matt Lopresti, a member of the Hawaiian House of Representatives, was at home when he received the alert on his mobile phone.
"We got our children, grabbed our emergency supplies, put them in our most enclosed room in our house which is our bathroom," he told local broadcaster KGMB.
"There's not much else you can do in that situation. You know, we did what we could... and I am very angry right now because it shouldn't be this easy to make such a big mistake."
Golfers competing at the US PGA Hawaii Open in Honolulu were also thrown into alarm, with US player Talor Gooch tweeting that "birdies didn't seem too important for a few minutes".
