Hawaii Governor David Ige has apologised for the "pain and confusion" caused by false ballistic missile attack alert.
In a conciliatory news conference, Ige promised to evaluate the testing system to ensure such a mistake would never happen again.
The alert on Saturday sent the islands into a panic, with people abandoning cars in a highway and preparing to flee their homes until officials said the cellphone alert was a mistake.
Hawaii Emergency Management Agency Administrator Vern Miyagi said the error happened when someone pushed the wrong button.
Both Miyagi and Ige promised a single person will not be able to make such an error in the future.
The emergency alert, which was sent to cellphones just before 8:10am, said in all capital letters: "Ballistic missile threat inbound to Hawaii. Seek immediate shelter. This is not a drill."
The Hawaii Emergency Management Agency tweeted that there was threat about 10 minutes later. But a revised push alert stating there was no threat went out sometime after that.
Agency spokesman Richard Repoza confirmed it was a false alarm and the agency is trying to determine what happened.
The incident 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".
Michael Kucharek, spokesman for the North American Aerospace Defense Command in Colorado Springs, Colorado, said NORAD and the US Northern Command are still trying to verify what happened in Hawaii - but that "NORAD did not see anything that indicated any sort of threat to Hawaii."
The White House said President Donald Trump, who is in Florida, was briefed on the false alert. White House spokeswoman Lindsay Walters said it "was purely a state exercise."
At the PGA Tour's Sony Open on Oahu, Waialae Country Club was largely empty and players were still a few hours from arriving when the alert showed up.
The tournament staff urged the media centre to evacuate. Staff members at the club streamed into the clubhouse and tried to seek cover in the locker room, which was filled with the players' golf bags, but instead went into the kitchen.
Hawaii US Senator Brian Schatz tweeted the false alarm was "totally inexcusable" and was caused by human error.
"There needs to be tough and quick accountability and a fixed process," he wrote.
Federal Communications Commission Chairman Ajit Pai also took to social media to announce the panel would launch an investigation.