After fire erupts at California energy facility, investigators look for answers

California officials are investigating the cause of a fire this week at an energy facility that brought a major interstate highway to a halt and forced thousands of residents to shelter in place for hours, including the possibility that it was sparked by the 4.5 magnitude earthquake on Monday.

“I’m confident our investigators can work around the structure to determine the cause of the fire,” said Contra Costa County Fire Department Public Information Officer Steve Hill, whose department is leading the investigation. He added: “Anything related to the earthquake would be speculation at this point."

The inferno, which began Tuesday afternoon at the NuStar Energy plant in Crockett, involved two large tanks that held ethanol, an additive used for vehicle fuel. The tanks contained at least 250,000 gallons, according to the Contra Costa Fire Department. NuStar said that that volume makes up only about 1% of the tank capacity.

One of the tanks exploded, sending its roof flying into the air and then crashing to the ground. Nearby residents said the explosion was so powerful that they thought it was a second earthquake on the heels of Monday’s 4.5 tremor, which hit around 15 miles from the energy plant.

Thick black smoke rose through the air, impeding the work of first responders. A related vegetation fire also broke out in the area, affecting approximately 14 acres.

Immediately following the outbreak, NuStar enacted it’s emergency response procedures, which includes cooling adjacent tanks “to minimize the risk of the fire spreading” and contacting all regulatory agencies, the energy facility said in a statement.

Contra Costa Fire, which responded to the alarm, used water and foam to fight the blaze and has since been keeping foam blankets to smother the fire and prevent oxygen from getting to the ethanol. These blankets also help to protect flareups so nearby tanks, which hold ethanol and jet fuel, don’t catch fire too. Officials say these tanks are being examined to ensure structural integrity.

“We’re examining one in particular,” said Hill.

“We believe that it is safe. We want to make absolutely sure and so that assessment team is in there looking at that one tank right now. The rest of the tanks we were concerned about all day yesterday, we determined they maintained their structural integrity. They are safe,” he continued.

While there were no employee injuries, one NuStar employee was unable to evacuate and was forced to hide in a culvert until he was rescued by first responders. One firefighter, however, did sustain minor injuries.

The fire, which shut down both directions of Interstate 80, caused major delays during the rush hour period and forced frustrated drivers to seek alternate routes. The highway eventually reopened late Tuesday night.

Residents in surrounding areas were forced to shelter in place for several hours, including children who were forced to shelter in schools as the air conditions were not safe enough to allow parents to pick up their kids. By Wednesday though, officials say there is no ongoing public health threat and the shelter in place had been lifted.

“One of the things that the assessment team that is in the facility right now is doing is they are looking at a solution of mitigating the vapor emissions that are coming off of both of those where the tanks location were,” said Hill.

“What’s happened is the two tank’s that burned were more or less destroyed in the fires and the secondary containment facilities which are earth and berms around those tanks are now holding the remainders of the materials that were in the tanks,” he added.

ABC News' Jenna Harrison, Marilyn Heck, Alyssa Pone and Timmy Truong, Leah Larosa and Will Carr contributed to this report.