After issuing the alert on Baekryeongdo, the home ministry left it to regional governments to decide whether to follow suit, according the Seoul city government. Officials in Seoul said they decided to issue an alert in the city as a precaution, even if they had to retract it. The mayor of Seoul later issued a public apology.

For Chung Sung-hee, 62, the confusing response was infuriating. Ms. Chung said she was preparing breakfast at her home in central Seoul when she heard the phone alert, followed by a loudspeaker broadcast. When she opened the window and trained her ears, all she could make out was that it was “a real situation,” not a drill.

“They should’ve said what was happening, and where to go,” Ms. Chung said. “Who would evacuate with a message like that?” When she got the second alert saying it was a false alarm, Ms. Chung said she couldn’t help but curse the authorities.