The incident happened at about 12:25 a.m. local time on Tuesday when a woman working at local radio station KSTO AM activated the Emergency Alert System (EAS), resulting in the broadcast of a Civil Danger Warning.
"Emergency Alert System: Civil Danger Warning," said the message that appeared on all of the island's TV channels. "An EAS Participant has issued a Civil Danger Warning for the following Guam counties: Guam. Effective until August 15, 12:40 AM ChST."
A real warning would normally include information about the threat and the recommended course of action, but a Civil Danger Warning is a rare type of alert which is used to warn of danger to a significant civilian population. Examples include an imminent or in-progress military or terrorist attack.
The mistake startled many viewers, prompting some of them to call police or the Office of Homeland Security in an attempt to get information. Guam Homeland Security Advisor George Charfauros said the agency had received "a lot" of phone calls.
There were conflicting reports on whether the message was also broadcast on local radio stations, but Charfauros said the island's emergency sirens had not been activated. "Many people were asleep, fortunately," he said.
"The unauthorized test was NOT connected to any emergency, threat or warning," Guam Homeland Security and Civil Defense (GHS/OCD) said in a brief statement. "GHS/OCD has worked with KSTO to ensure the human error will not occur again."
Homeland Security Advisor Charfauros said an unauthorized employee at KSTO AM had caused the system to be activated, but he provided no further information, except to say that the action is not believed to have been intentional.
Guam has been at the center of attention for days after North Korea threatened to launch missiles towards the U.S. territory, which is home to 162,000 Americans and the Anderson Air Force Base. Although the North's threat is to launch missiles into the water, it has nonetheless raised tensions.
"Residents and visitors are reminded to remain calm, even with the continued unconfirmed reports throughout the media," Charfauros said in a statement. "Remember there is no change in threat level, we continue business as usual and know there are U.S. Department of Defense capabilities in place."
Charfauros added: "We continue communication with our federal and military partners and have not received official statement warranting any concern for imminent threat to Guam or the Marianas."