Residents of Hawaii were told that a ballistic missile was heading for the Pacific island and that they should take cover, in a emergency alert sent to people's phones entirely by accident.
The message, which also interrupted TV broadcasts, sparked panic before a second alert sent around 30 minutes later correcting the error.
"Ballistic missile threat inbound to Hawaii," the original message read. "Seek immediate shelter. This is not a drill."
A second alert was sent that read: "There is no missile threat or danger to the State of Hawaii. Repeat. False alarm."
Officials in the country quickly ruled out hacking and have said that human error was to blame during a routine test of its systems during a shift change.
The snafu comes amid heightened tensions between the US and North Korea, which has been sanctioned over missile tests.
The chair of the US Federal Communications Commission Ajit Pai said there would be a full investigation into the incident.
"This system we have been told to rely upon failed and failed miserably today," said House of Representative speaker Scott Saiki. "Measures must be taken to avoid further incidents that caused wholesale alarm and chaos today."
According to recent reports, the country's emergency alert system which is delivered by the FCC with telecoms and tech companies and used by local government and emergency services to send alerts, is due to be overhauled.