Firefighters responded to a police call at 480 Easy Street at about 1:15 p.m. Thursday. The business, called Realm Industries, is an alternative fuel company.
Three workers were inside at the time of the explosion. Two managed to get out, but an adult male was killed in the blast.
Firefighters located the body in the partially collapsed building, determined there was nothing that could be done for the person and then backed out to assess what they were dealing with, Oatman said.
Fire officials have not determined what caused the explosion. Firefighters were carefully moving back inside, checking to make sure there was no explosive atmosphere.
"We're slowly going in, doing some monitoring, and we're ruling out that we don't have any type of flammable of explosive atmosphere," said Oatman.
Businesses in a half-mile radius were evacuated after blast, which blew a hole in the building's roof.
People in the area say they not only heard the blast, but they felt it.
"My heart still hasn't acclimated from the explosion, it's definitely a sound that I'll take to my grave and will never forget. Just looking up and seeing the debris fly and the smoke after that sound, [it's] something never I'll never forget," said Simi Valley resident Brian Westerhouse.
Workers in the surrounding area saw a shower of debris after the blast.
The Associated Press contributed to this report.