The Hawaii emergency management worker, fired earlier this week after nearly 12 years on the job, said he did not hear someone repeating the word "exercise" before the January 14 drill because it wasn't played over a speaker, CNN reported.
"The protocol is that the person answering the call presses the speaker phone button so everyone in office can hear the message".
The almost 40-minute delay in standing down the alert sparked recriminations, with state officials apologising.
The employee also reportedly had at least two previous incidents in which he mistook drills for real-world events, including for tsunami and fire warnings.
He added: "I feel very badly for what's happened - the panic, the stress people felt, all the hurt and pain". I felt sick afterward.
Speaking to NBC News, the former employee of the Hawaii Emergency Management Agency (HI-EMA) instead blamed a systemic failure for the false alert on January 13 that sent thousands of terrified residents scrambling for cover.
"I was 100 percent sure that it was real and I did what I was trained to do", he said.
An investigation released by the state described the employee as having a poor work history; other members of its staff said they did not feel comfortable with his work.
The man said he didn't hear the beginning of the message that said, "exercise, exercise, exercise".
"When the phone call came in, someone picked up the receiver instead of hitting speakerphone so that everyone could hear the message", he said.
He added that he heard "this is not a drill", but never the word "exercise", which would have indicated that it was, indeed, a practice run.
But HI-EMA didn't send a retraction to cellphones until 38 minutes after the original alert.
A Federal Communications Commission report found that the emergency worker did in fact actually believe the threat was real. Since then, he's reportedly been distressed to the point that he has had trouble eating and sleeping.
The fired employee insisted he's being scapegoated by Hawaiian officials and he is considering legal action against The Aloha State. The state did not name him.
In an interview with multiple news outlets for the first time, the man who accidentally triggered widespread panic said he's been made a scapegoat by Hawaii Emergency Management and the state. Another employee was being suspended without pay, officials said.
But he said that he thinks the military is better equipped for alerts.
A Hawaii state department of defense spokesman, Lt Col Charles Anthony, declined to comment on what the former worker said.
According to federal officials, he has refused to co-operate with investigators beyond submitting a written statement. He wasn't trying to impede any investigations, he said: "There really wasn't anything else to say".