Television broadcasts and mobile phones in Hawaii were interrupted by an emergency warning of an incoming missile on Saturday.
The message sent to mobile phones warned, in capital letters: Ballistic missile threat inbound to Hawaii. Seek immediate shelter. This is not a drill.
A video on social media showed the emergency system interrupting the broadcast of a football match, with a high-pitched sound alerting viewers to their screens which displayed the same warning.
The message, which was sent at 8.07am local time (6.07pm GMT), was followed by a retraction 38 minutes later stating the missile alert was a false alarm.
But while the state emergency management agency was struggling to retract the alert, Hawaiians scrambled to find shelter in frenzied scenes.
On the H-3, a major highway north of Honolulu, vehicles were left abandoned after drivers ran to a nearby tunnel, local media reported.
Professional golfer Colt Knost, staying at Waikiki Beach during a PGA Tour event, said everyone was panicking in the lobby of his hotel.
Everyone was running around like, ‘What do we do?’ he said.
One father even made his daughter climb down a manhole in order to seek refuge.
David Ige, the governor of Hawaii, said the accidental alert was unfortunate and regrettable and apologised to Hawaiians.
It was a mistake made during a standard procedure at the change over of a shift, and an employee pushed the wrong button during a shift change, he explained.
A spokesperson for the White House said that President Trump had been briefed on the alarm. This was purely a state exercise, they said.
House Speaker Scott Saiki said in a statement: Clearly, government agencies are not prepared and lack the capacity to deal with emergency situations.
Hawaiian Senator Brian Schatz said on Twitter: It was a false alarm based on a human error.
What happened today is totally inexcusable. The whole state was terrified. There needs to be tough and quick accountability and a fixed process.
The message seems to have been sent as part of the US Wireless Emergency Alerts (WEA) system, which allows authorised national, state or local government authorities to send alerts regarding public safety emergencies.
Last December, the Pacific island tested sirens warning of an impending nuclear attack from North Korea.
Hawaii had been named as a potential target of an intercontinental ballistic missile launched by Kim Jong Un, who has threatened the US with a nuclear strike.
Officials in Hawaii have spent months briefing the public on what action to take in the event of an attack. It is estimated that Hawaiians would have less than 20 minutes before the missile arrived.