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.”
Hawaii Emergency Management Agency later confirmed there was no threat.
The 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.
It is still unclear how the initial mistake was made.
‘Full blown panic’
The Honolulu Star-Advertiser says emergency officials mistakenly sent the message out by text, all in capital letters, at 08:07 (18:07 GMT) before correcting the error by email 18 minutes later.
There was no follow-up mobile text until 38 minutes after the original alert, it said.
In a tweet, the state’s Emergency Management Agency (EMA) said simply: “NO missile threat to Hawaii.”
The agency describes reaction in the state as “full-blown panic”.
Jamie Malapit, owner of a Honolulu hair salon, texted clients to say he was cancelling their appointments and closing his shop for the day, AP reports.
He said he had still been in his bed when his phone started ringing “like crazy”.
Afterwards, he was still “a little freaked out” and feeling paranoid even after hearing it was a false alarm, AP adds.
The alert came amid a growing threat from North Korea’s missile and nuclear programme. Hawaii is the closest US state to North Korea.
In September Pyongyang carried out its sixth nuclear test.
Last month, the Star-Advertiser also reported that a missile launched from North Korea could strike Hawaii within 20 minutes of launch