An evacuation order remains in effect for some 50,000 area residents with no timeline on when they can return, fire officials said on Sunday.
TJ McGovern, the interim fire chief for the Orange County Fire Authority, said in a video posted on social media that fire officials were able to evaluate the tank more closely overnight and spotted a potential crack that could be relieving some of the pressure inside.
But he cautioned that the information is still being vetted and validated.
"With this new information, it could change our trajectory and our strategy to this event," McGovern said.
"Last night, this operation that we did gave us positive intel to make educated decisions today in the positive light. We're not there yet, but this was a step in a right direction."
Orange County Fire Authority Captain Wayhowe Huang told The Associated Press earlier Sunday that it does not appear that any of the highly volatile chemicals in the tank have leaked.
"There's still the danger of a possible explosion. We're not taking that off the table," Huang said.
"We're still operating as if that is the risk."
Firefighters have been spraying the outside of the tank with water in an effort to cool the chemicals inside and prevent an explosion.
The pressurised tank overheated on Thursday and began venting vapours at a company site in Garden Grove, about 60 km south of downtown Los Angeles, according to the Orange County Fire Authority.
A crack could be a welcome development, as it could mean product or pressure inside the tank is being released, reducing the chance the tank explodes, said Andrew Whelton, an engineering professor at Purdue University.
"Think of a soda can. If you leave it in a hot car it can explode," he said.
"But if you put a hole in the can, the product is released and the can itself doesn't explode."
But Faisal Khan, head of the chemical engineering department at Texas A&M University, said a crack suggests an explosion could still happen -- just not in the magnitude initially feared.
"Cooling is happening on the surface of the tank while runaway reaction may be occurring deep inside the tank," he explained.
"Yes, reaction is slowed compared to what it started. However, we are not out of explosive release risk."
Meanwhile, some Garden Grove residents filed a class-action lawsuit on Saturday against GKN Aerospace Transparency Systems, the company that operates the facility where the tank is located.
Lawyers for residents living in the evacuation zone argued in their federal court lawsuit that regardless of what happens next, property values in the surrounding community are sure to be impacted.
Spokespersons for the company didn't comment on the lawsuit itself, but pointed to a Saturday statement in which they apologised to residents and businesses that have been forced to evacuate.
On Sunday, the company released another statement saying it was monitoring the "condition of the affected material" and "working around the clock to mitigate the risk of a leak".
The weather will be an important factor in determining where a plume of chemicals would go in the event of an explosion.
Officials were developing maps to predict different scenarios about which areas would be most affected.
Garden Grove is next to Anaheim, home to Disneyland's two theme parks, which were not under evacuation orders.
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