The explosion happened at a company called Rainbow of Hope on the 12300 block of Gladstone Avenue Tuesday afternoon.
The Ventura County Fire Department was asked to join in the investigation because of a similar explosion that occurred in Simi Valley in 2010 that killed one person. Last year's explosion occurred at a business owned by the same family that owns the Sylmar business.
Tyson Larson was killed in the June 2010 blast in Simi Valley. His older brother Timothy was seriously injured in Tuesday's blast in Sylmar.
The Larson family runs the alternative-energy-fuel business Realm Industries.
The California Division of Occupational Safety and Health (CAL OSHA) cited the company for problems related to the inspection and maintenance of a pressure vessel and the lack of a permit for an airtank.
The Sylmar business, Rainbow of Hope, passed inspection in May to operate at the location.
"We've asked them to come because of their expertise having had prior experience with such an investigation to assist us as well as CAL OSHA in this investigation," said Los Angeles City Fire Department Captain Jaime Moore.
According to friends and firefighters, Timothy Larson is an off-duty firefighter on disability leave. He is one of two people seriously injured in Tuesday's blast. He lost an arm and a leg in the explosion.
The cause of Tuesday's blast was under investigation.
The company was working on a process to create hydrogen fuel from water.
Larson and another employee remained hospitalized in serious condition Wednesday.
According to CAL OSHA, the investigation could take weeks to complete.
Bob Williamson with Wil Power Battery said the brothers were working on a new technology to create clean fuel from water.
"It concerns me because they had this explosion out in Simi, killing Tyson, and now this explosion, and they were very cautious. So I'm kind of surprised that this happened," said Williamson, who said Tim Larson demonstrated the technology for him and it seemed promising.
Authorities are focusing on hydrogen gas as the source of the explosion.
"It is our understanding this is a business that processes fuels from water, and that alternative fuel is apparently hydrogen, and somewhere in the process, an explosion occurred," said Los Angeles City Fire Department Assistant Chief Tony Varela.
Authorities said the force of the blast and the debris it launched into the air could have led to many more injuries.
The building owner said Larson assured him nothing dangerous was being done at the business.