In an interview with ABC News, the worker - who spoke on the condition of anonymity out of fear for his safety - said he never heard the defense drill described as such while it happened and that he explicitly heard the phrase "this is not a drill."
"I was 100 percent sure that it was real," said the man, who had been with his department for more than 11 years. "I did what I was trained to do."
Unlike previous exercises, this one was unannounced and happened when there was no supervisor present, he said. Throughout the ordeal, his colleagues were quiet and did not outwardly indicate that they believed the warning was part of a drill.
"I later found out that one of my coworkers thought it was real, but of course it was a drill call from our supervisor in the hallway," he said.
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When he thought the threat was real, the worker said he felt a "terrible feeling of dread" and was "very emotional afterward."
In the weeks since the Jan. 13 incident, the worker said he has received death threats, with his lawyer adding that they are still deciding whether or not to sue the state.
He said he feels guilty about what happened and empathizes with members of the public who were sent into a frenzy by the alert.
According to the Associated Press, 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.
His supervisors counseled him but kept him for a decade in a position that had to be renewed each year.
The ex-worker disputed that, saying he wasn't aware of any performance problems.
The Associated Press contributed to this report.