Tens of thousands of people were ordered to leave their homes in California on Friday after a huge chemical tank began to leak, sending toxic fumes over a heavily populated area and posing the risk of an explosion.
The tank contained 7,000 gallons (26,000 liters) of methyl methacrylate, a volatile and flammable liquid used to make plastics, with firefighters warning the situation was serious.
"There are literally two options left," Incident Commander Craig Covey said.
"The tank fails and spills a total of about 6-7,000 gallons of very bad chemicals into the parking lot in that area or, two, the tank goes into a thermal runaway and blows up, affecting the tanks around them that have fuel or chemicals in them as well.
"We are setting up these evacuations in preparation for these two options: it fails, or it blows up," AFP quoted him as saying.
The incident unfolded in the Garden Grove area of Orange County, southeast of Los Angeles.
Garden Grove Police Chief Amir El-Farra said about 40,000 people were affected by the evacuation order, with several thousand refusing to leave their homes.
Aerial footage filmed by local TV stations showed jets of water being sprayed at the tank, which has a capacity of 34,000 gallons.
Covey said later Friday that efforts to cool the tank had been successful.
"It's down to a temperature around 61 degrees, with 50 being its happy place so those efforts are succeeding," Covey said in a video update.
"Our group is going to do everything they can to come up with a third, a fourth, a fifth option," he added.
Orange County health officer Regina Chinsio Kwong said the large exclusion zone around the tank was a necessary precaution.
"If it does explode and there is a vapor, you are all safe as long as you are out of the zone that was determined to be an evacuation zone," she said.
She appealed for anyone who might notice "a fruity and heavy smell" to alert authorities.
"Smelling it doesn't mean you've reached a level that causes symptoms. But we don't want you to smell that. So we need to know if you're smelling it."
No injuries had been reported by Friday evening, and there was no immediate indication as to what caused the leak, which was initially reported on Thursday.
Covey said crews were preparing for a chemical spill, which he described as a "best-case scenario" and far preferable to an explosion and toxic plume.
Responders were working to put containment barriers in place to prevent any spilled material from reaching storm drains or river channels that funnel into the ocean.
The US Environmental Protection Agency says methyl methacrylate is irritating to the skin, eyes, and mucous membranes in humans.
"Respiratory effects have been reported in humans following acute (short-term) and chronic (long-term) inhalation exposures," a fact sheet on the agency's website says.
"Neurological symptoms have also been reported in humans following acute exposure."
