Authorities responded to 12349 Gladstone Ave. about 4:30 p.m. Two people were taken to a local hospital in critical condition and a third person was treated for minor injuries.
One of them was identified as Los Angeles City Firefighter Tim Larson, who lost an arm and a leg in the blast. His family is a partial owner of the business indentified as Rainbow of Hope, an alternative fuel company.
Officials say Larson was on disability.
Authorities said the explosion at the single-story, 200-foot by 100-foot commercial structure may have been caused by two compressed natural gas cylinders that likely exploded.
"It is our understanding that this is a business that processes fuels from water and that alternative fueld is apparently hydrogen, and somewhere in the process an explosion occurred," Los Angeles City Fire Asst. Chief Tony Varela said.
More than 100 firefighters searched the building and found no one trapped inside.
Some neighboring businesses were also damaged.
People who work nearby said the force of the explosion shook their buildings.
'We just saw a bunch of debris coming down from the sky," said witness Tony Coca.
This isn't the first tragedy for the Larson family. Last June, at a warehouse in Simi Valley where they were running an alternative fuel company, an explosion killed Larson's 28-year-old brother, Tyson.
The owner of the building says he was told the company was trying to find a way to achieve safe drinking water for third world countries and that they would not be doing the dangerous extraction process on his property.
"We heard they had a problem in Simi and we questioned them about it. We were told they had other properties where they did that activity," said building owner Dick Werth.