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.”
Local officials confirmed there was no threat and the US government announced there would be a full investigation.
The US military also confirmed the alert had been sent “in error”.
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.
How was the alert released?
The phone message, all in capital letters, went out at 08:07 (18:07 GMT).
In a tweet, the state’s Emergency Management Agency (EMA) said simply: “NO missile threat to Hawaii.”
Television and radio broadcasts across the state were also interrupted with a recorded emergency message: “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!”
How did Hawaiians react?
AP describes reaction in the state as “full-blown panic” while according to the CNBC news channel, the alert “momentarily put recipients into a state of frenzy, with scores reportedly running for shelter”.
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.
What is being done to prevent this happening again?
Ajit Pai, chairman of the US Federal Communications Commission, announced the investigation.
“The @FCC is launching a full investigation into the false emergency alert that was sent to residents of Hawaii,” he tweeted.
Why was Hawaii on edge before this?
North Korea’s missile and nuclear programme is seen as a growing threat to America.
In September Pyongyang carried out its sixth nuclear test.
Last month, the Star-Advertiser reported that a missile launched from North Korea could strike Hawaii within 20 minutes of launch.
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