The victims were identified as Jose Jimenez, a 51-year-old from Garden Grove, and Isidro Echeverria, a 34-year-old from Oceanside.
Cal Osha is now handling the investigation.
Authorities have narrowed down the cause of the blast to failure of some sort of industrial-strength water heater.
"It was a large water heater, it was an electric water heater, not a gas water heater, and somehow or another, it failed and caused an explosion," said Capt. Greg McKeown from Orange County Fire.
The powerful blast blew a hole in a concrete wall and shook a fire station across the street at about 11:40 a.m.
"We were just working at a machine, and suddenly we just heard a boom, loud, and the door just slammed," said witness Alvaro Morales.
Authorities said the company called Solis makes plastic parts for conveyor belts, and there was a lot of equipment inside the building.
The blast was heard for blocks.
"The first thing that came to my mind, somebody threw something through the door, but then we came out," he said.
There is extensive damage inside the building, and authorities said a lot more people could have been hurt.
Firefighters at the fire station across the street from the building said the explosion shook them from their sleep.
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