HONOLULU — If there’s a ballistic missile strike from North Korea, Hawaii will be faced with thousands of casualties, thermal radiation, severe damage to critical infrastructure, widespread structural fires and other chaos, the administrator of the Hawaii Emergency Management Agency said Thursday at an informational briefing in the state Capitol.

The picture Vern Miyagi painted is for a threat he said is not likely to happen. Still, it’s a threat Hawaii can’t ignore, he said during the briefing organized by state lawmakers.

Hawaii lawmakers have been urging emergency management officials to update Cold War-era plans for coping with a nuclear attack as North Korea develops nuclear weapons and ballistic missiles that can reach the islands.

Honolulu would be the most likely target but impact on a neighbor island cannot be ruled out, Miyagi said. Officials believe there would be up to 15,000 casualties, he said.

Once a missile is launched, there will be less than 20 minutes’ warning, Miyagi said.

That leaves little time to do much more than get inside and stay tuned, he said, adding that he’s been criticized for that simplistic guidance. “That’s it?” people ask, he said. “But when you have 20 minutes this is what you have to do.”

A woman in the audience asked what people should do if they’re driving when words comes about an attack. Get out and lay flat on the ground, or try to get near a concrete structure, said Emergency Management Agency Executive Director Toby Clairmont, adding that the roads would be chaotic.

Ewa Beach resident Nick LaCarra asked about the lack of shelters in Hawaii, despite the state’s vulnerability to hurricanes. When the Cold War ended, funding for maintaining shelters ran out because there was no longer any threat and there’s no funding to re-establish them, Miyagi responded.

Hawaii does have an “attention-alert” steady tone siren with which residents are already familiar. The steady tone siren signals residents to tune into the radio or TV for information on emergencies such as a tsunami.

This year, the Emergency Management Agency announced that Hawaii will reintroduce monthly tests of an “attack-warning” siren that the state hasn’t heard since the end of the Cold War.

The steady tone siren is tested on the first working day of each month. Officials proposed adding the wailing siren on the same day as early as November, but are holding off to prevent confusion.