The former Hawaii state worker who sent a false missile alert last month that prompted mass panic across the islands has said he was "100% sure" the attack was real.

Speaking to reporters on the condition of anonymity, the man said the call he received on January 13 did not sound like a drill. State officials have subsequently said other workers clearly heard the word "exercise" repeated several times.

The man in his 50s, who was fired by the Hawaii Emergency Management Agency, said it felt like he had been hit with a "body blow" when he realised it was just a drill and he has had difficulty eating and sleeping since.

The man's superiors said they knew for years that he had problems performing his job. The worker had mistakenly believed drills for tsunami and fire warnings were actual events, and colleagues were not comfortable working with him, the state said.

The ex-worker disputed that, saying he was not aware of any performance problems.

While working at the state warning site in a former bunker in Honolulu's Diamond Head crater on January 13, the man said, he took a call that sounded like a real warning from US Pacific Command. He said he did not hear that it was a drill.

But the problems at the agency went beyond the one employee.

Federal and state reports said the agency had a vague checklist for missile alerts, allowing workers to interpret the steps they should follow differently. Managers did not require a second person to sign off on alerts before they were sent, and the agency lacked any preparation on how to correct a false warning.

Those details emerged on Tuesday in reports on investigations about how the agency mistakenly blasted mobile phones and broadcast stations with the missile warning.

It took nearly 40 minutes for the agency to figure out a way to retract the false alert on the same platforms it was sent to.

"The protocols were not in place. It was a sense of urgency to put it in place as soon as possible. But those protocols were not developed to the point they should have," retired Brigadier general Bruce Oliveira, who wrote the report on Hawaii's internal investigation, said at a news conference.

Hawaii Emergency Management Agency administrator Vern Miyagi resigned as the reports were released. Officials revealed that the employee who sent the alert was fired on January 26. The state did not name him.