A US watchdog agency launched an investigation Saturday into the "near-disaster" fire at oil giant Chevron's Richmond, California refinery that nearly killed workers.
The August 6 fire was sparked when "gas-oil" -- a 600-degree Fahrenheit (315 Celsius) combustible hydrocarbon liquid -- leaked from a pipe, and ignited as workers were trying to repair it.
No one was killed, but "witness testimony collected by CSB investigators indicates that a large number of workers were engulfed in the vapor cloud," team leader Dan Tillema of the Chemical Safety Board said in a statement.
"These workers might have been killed or severely injured, had they not escaped the cloud as the release rate escalated and the cloud ignited, shortly thereafter."
CSB head Rafael Moure-Eraso said the event was "among the most serious US refinery incidents in recent years."
Industrial security experts on Monday will join a team of seven investigators who have been on site since two days after the incident.
"We agree that this is a serious incident that warrants thorough investigation," Chevron spokesman Justin Higgs told AFP.
"We are cooperating with all regulatory agencies and are committed to better understanding the root cause of this incident."
The spokesman emphasized that the oil giant's employees stayed safe because of "their strict adherence to emergency procedures in evacuating the area when it became clear that the situation was escalating."
According to the CSB statement, investigators will focus on why the leaking pipe was not replaced during a round of maintenance in late 2011. Chevron would not comment directly on the leaking pipe.
Higgs said the Richmond refinery is online, though at a reduced capacity, and only the equipment that had been involved in the fire remains shut down.