The fire was first reported around noon on Monday as firefighters saw flames from the building at 8423 N. Canoga Ave.
According to an update issued by the Los Angeles Fire Department, one man was found dead Tuesday morning. A spokesperson for the fire department said three other men suffered "severe" burn injuries.
One of them suffered burns to 90% of his body, according to LAFD, and died due to his injuries. He was 28 years old.
The remaining two men are still in critical condition.
At least 150 firefighters battled the blaze at the one-story commercial building. Investigators reported seeing six vehicles parked near the building that were damaged.
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Witnesses described hearing at least one explosion, and seeing at least one man on fire as he emerged from the building.
One woman said her husband was in the area at the time and saw the man come out on fire.
"He described there was a gentleman who came out fully engulfed, on fire," she said. "He got the fire extinguisher, put the gentleman out and helped to drag him to safety."
It's unclear if that man was the person found dead during Tuesday's investigation.
Firefighters declared a knockdown 75 minutes after they arrived, and they stayed on scene to monitor hot spots and assist with the investigation.