Tommy Freeman of Kailua, Hawaii, said that after seeing the alert, he was “overcome with adrenaline — we thought we had 15 minutes to live.”
Traffic stopped, he said, with people running around in fear. “It was a weird pandemonium,” he said. “You could tell no one knew what to do.”
The false alarm caused widespread panic on social media.
It was not immediately clear why the false alert was sent. Gov. David Y. Ige, a Democrat, said in a statement that he was working to figure out what had happened.
“While I am thankful this morning’s alert was a false alarm, the public must have confidence in our emergency alert system,” he said. “I am working to get to the bottom of this so we can prevent an error of this type in the future.”
The White House confirmed that President Trump had been briefed.
Senator Mazie K. Hirono, Democrat of Hawaii, emphasized the importance of making sure that “all information released to the community is accurate.”
She added, “We need to get to the bottom of what happened and make sure it never happens again.”
Senator Brian Schatz, Democrat of Hawaii, tweeted a similar statement. “There is nothing more important to Hawaii than professionalizing and fool-proofing this process,” he said.
“What happened today is totally inexcusable,” he added. “The whole state was terrified.”
Angel Kay Uherek, who lives in Kailua, said that soon after seeing the alert, she began packing a bag, and she messaged her five children on the mainland to let them know of the threat.
“I’ve been here 13 years and we’ve had tsunami warnings,” she said. But with tsunami threats, “There’s more time than 15 minutes.”
And after recent standoffs between Mr. Trump and the North Korean leader, Kim Jong-un, she said, the threat of nuclear attacks is at the top of everyone’s mind.
“It just got real,” she said. “It’s not a joke.”
Wren Wescoatt, who lives in Manoa Valley, a residential area near the University of Hawaii, said he was sleeping in when he received the alert, and he immediately began calling relatives on neighbor islands.
“You wonder: What does seek shelter mean if we come under attack? What can we do?” he said. “My wife’s first thought was, do we have enough water? But does that really matter if a missile strikes Hawaii? It might be better to just hug the kids.”