Emergency officials say a message warning of an incoming incoming ballistic missile to Hawaii was a mistake.
The emergency alert, sent to all mobile phones shortly after 8am on Saturday, said in all caps, "Ballistic missile threat inbound to Hawaii. Seek immediate shelter. This is not a drill".
The alert was also sent to TV and radio stations, who broadcast the warning.
Hawaii Emergency Management Agency spokesman Richard Repoza said it was a false alarm.
The alert stirred panic for residents on the island and across social media.
Video shared on Twitter showed one man trying to place his children in a storm water drain to take shelter.
Hawaiian Governor David Ige told CNN an employee had "pushed the wrong button".
The EMA and the North American Aerospace Defense Command (NORAD) both said they were investigating how the alert was sent out.
The incident also prompted defence agencies including the Pentagon and the US Pacific Command to issue the same statement, that they had "detected no ballistic missile threat to Hawaii".
A White House spokeswoman said President Donald Trump, who is spending the weekend in Florida, has been briefed on the incident.
The EMA later sent a tweet saying there was no missile threat.
Last month the EMA conducted a siren drill to warn the public of a possible nuclear attack from North Korea.
The US state was the first to test a Cold War era nuclear warning system since the end of the Cold War.