A former Hawaii state worker who sent a false missile alert last month said on Friday that he was devastated for causing panic but was "100 percent sure" at the time that the attack was real.

The man in his 50s spoke to reporters on the condition that he not be identified because he fears for his safety after receiving threats.

He says the on-duty call he received on January 13 didn’t sound like a drill.

"I heard: 'This is not a drill.' I didn't hear 'exercise' at all. I'm really not to blame in this. It was a system failure," he told NBC Nightly News.

However, state officials say other workers clearly heard the word "exercise" repeated several times.