Tens of thousands of Southern California residents were evacuated after officials warned a chemical tank holding a "highly flammable" substance at an aerospace facility could inevitably fail and potentially explode.

And while officials said Monday that the threat of a massive explosion was "off the table" after an overnight operation, the threat of toxic chemicals leaking out of the tank or a small explosion remained possible.

"It's still a potential," Orange County Fire Authority public information officer Nick Garton told CBS News Los Angeles on Monday. "We're trying to confirm that there's still a liquid form of this chemical that could potentially leak out."

The tank at GKN Aerospace is estimated to contain 7,000 gallons of methyl methacrylate, a "highly volatile" substance used in the production of plastics, according to Orange County Fire Authority incident commander Craig Covey. 

The tank at the center of the incident began overheating Thursday and started venting vapors, prompting firefighters to work to cool off the chemical inside to prevent a failure that could result in an explosion.

"Positive news" amid ongoing threat of liquid leak

Orange County officials announced on Monday morning that the threat of a BLEVE, or Boiling Liquid Expanding Vapor Explosion, had been eliminated following an operation overnight.

"We are happy to report that the threat of a BLEVE is now off the table," Orange County Fire Authority interim Chief TJ McGovern said. "That threat has been eliminated."

In a social media post, McGovern and Covey said crews confirmed a crack on the tank, which led to a pressure release. The tank's internal temperature has decreased from 100 degrees to 93 degrees.

"That is incredibly positive news as we turn the corner on this incident," Covey said.

Garton made it clear Monday there was no active leak, but said more cracks could form at the bottom of the tank. As pressure continues to lower, however, so do the chances of cracks forming, he said.

The Environmental Protection Agency was monitoring the area and said Sunday there weren't any signs of toxic chemicals being released into the environment.

Evacuations and state of emergency

Roughly 16,000 residents of surrounding areas were still under mandatory evacuation orders, officials said Monday night, down from over 50,000 a day earlier. The evacuation zone had decreased in size by about 65%, officials said.

McGovern urged people on Sunday night: "Please help us. Stay out of the evacuation zone. Let us run our operation."

California Gov. Gavin Newsom declared a state of emergency for Orange County on Saturday, saying he was "mobilizing every state resource available to support local responders and make sure the community has what they need to stay safe."

Newsom also asked President Trump on Sunday for a federal emergency declaration.

A White House official told CBS News on Monday that the Trump administration "is engaged and monitoring the situation in Garden Grove," where the GKN Aerospace facility is located, noting that federal resources were assisting with the response.

"The U.S. EPA has integrated with the local Unified Command composed of state and local agencies, and has enabled air monitoring at 20 locations around the area," the White House official said. "FEMA has deployed a Liaison Officer to coordinate with officials and has also deployed a team to the State Emergency Operation Center to support incident contingency planning. FEMA has also activated the Interagency Modeling and Atmospheric Assessment Center to provide plume modeling of airborne hazards, informing incident leadership and contingency planning." 

Methyl methacrylate 

Methyl methacrylate is a flammable substance that generates heat on its own. That means if it leaks into the atmosphere, any spark or flash could lead to an uncontrolled fire or explosion, said Elias Picazo, an associate professor of chemistry at the University of Southern California.

Before officials said on Monday that the tank's temperature was down, it had climbed in previous days, with crews finding that temperatures were increasing at about 1 degree per hour. It was at 90 degrees on Friday night after starting the day at 77, Covey said in a video posted to X on Saturday. As of Saturday night, firefighters said that the internal temperature of the tank was over 100 degrees, more than double the "happy place" for the substance.  

Earlier, Covey told CBS Los Angeles that this is "the most significantly dangerous event" he's been a part of in his decades in fire service. 

"This is bad as I've ever seen," he added. 

No injuries or deaths have been reported. GKN Aerospace is in Garden Grove, a town about 38 miles south of Los Angeles. 

Health risks of methyl methacrylate 

Methyl methacrylate is a respiratory irritant, health experts told CBS Los Angeles. Exposure to the chemical can cause lung, skin and eye irritation, nausea and dizziness. 

Covey said there are "two options" right now. In one scenario, the tank could fail and spill thousands of gallons of "very bad chemicals into the parking lot and that area," he said. 

Because the chemical is heavier than air, its vapor would settle and sink. Covey said crews have used sandbags to build containment barriers so that any spilled chemicals do not leak into storm drains or nearby waterways. 

In the second scenario, the tank could blow up, Covey had said. This would send methyl methacrylate particles into the air. Covey said it's unclear how widespread or dangerous such an explosion could be. An uncontrolled explosion would also affect surrounding tanks, which hold chemicals and fuel. 

Covey said he and other officials were working to determine other options to safely end the incident. 

"Letting this thing just fail and blow up is unacceptable to us," Covey said. "Our goal is to find something and not allow that to happen."