It's too soon to tell if human error led to the fire at the Valero McKee refinery.
Investigators are fairly certain liquid propane did.
The U.S. Chemical Safety Board says testimony suggests liquid propane released into the air, expanded to a vapor cloud and then ignited.
That led to what they are now calling a flash fire.
It's possible chlorine, a toxic gas, was also released, but that is still under investigation.
Lead investigator Don Holmstrom says it's so hazardous where the fire ignited, it's kept investigators about 200 feet away from the main affected area - but they are closing in on it.