Federal, state and local authorities in the United States continued emergency response operations Monday after a chemical incident at an aerospace facility in Garden Grove, Southern California, prompted the evacuation of nearly 50,000 residents across multiple communities in Orange County. Officials said the immediate threat of a catastrophic explosion had been eliminated, although evacuation orders remained in effect in affected areas.

According to the Orange County Fire Authority, the incident began Thursday at the GKN Aerospace facility in Garden Grove after a 34,000-gallon tank containing methyl methacrylate began overheating and venting chemical vapor. Methyl methacrylate is a highly flammable industrial chemical commonly used in the production of acrylic plastics.

Fire officials had warned that failure of the compromised tank could result either in the release of thousands of gallons of chemicals or a boiling liquid expanding vapor explosion, known as a BLEVE. Mandatory evacuation orders were subsequently issued for portions of Garden Grove, Anaheim, Buena Park, Cypress, Stanton and Westminster.

Emergency shelters operated by the American Red Cross and local authorities were reported to be near capacity as displaced residents awaited updates on when they could safely return home. Officials said shelters at Golden West College, Los Alamitos High School and the Orange County Fairgrounds continued operating Monday.

The Orange County Fire Authority said Monday that the threat of a worst-case BLEVE event had been eliminated after hazardous materials crews successfully reduced pressure inside the damaged tank during overnight operations. Officials said a newly discovered crack in the tank helped release pressure and lower the temperature of the chemical.

"We are happy to report that the threat of a BLEVE is now off the table," Orange County Fire Authority officials said in a post on X Monday morning. Officials added that crews were able to obtain a temperature reading of 93 degrees from the tank after earlier gauges had failed to measure temperatures above 100 degrees.

Authorities cautioned, however, that smaller ignition risks could still remain because flammable material was still present at the site. Continuous atmospheric monitoring around the facility had not detected active toxic plumes outside the evacuation perimeter, according to officials.

Orange County Fire Chief TJ McGovern said evacuation boundaries were originally established based on the potential blast radius of a worst-case explosion and were now being reevaluated following the reduced threat level.

"Now that that's off the table, we're going to evaluate the zones for an explosion," McGovern told KTLA. "So, in theory, the explosion zone is probably going to be reduced."

Authorities nevertheless urged residents not to return to evacuation areas until official clearance is provided. As of Monday afternoon, evacuation orders remained in effect while emergency crews continued stabilization operations.

California Governor Gavin Newsom declared a state of emergency for Orange County over the weekend and requested federal assistance. On Monday, US President Donald Trump approved California's request for a federal emergency declaration, authorizing the Federal Emergency Management Agency to provide personnel, equipment and other emergency support resources.

"We are grateful for the federal government approving our request and helping strengthen the resources already on the ground to help this community recover safely," Newsom said in a statement released Monday.

The incident has also prompted legal and regulatory scrutiny. Todd Spitzer, Orange County district attorney, announced that his office had opened an investigation into GKN Aerospace and called for whistleblowers with information regarding operations at the facility to come forward.

"For goodness' sake, they're in the middle of a commercial area, residential, it's an urban population," Spitzer said in comments reported by CBS News. "It's irresponsible, it's horrific, and I'm angry about it."

CBS News also reported that GKN Aerospace reached a settlement last year with the South Coast Air Quality Management District over allegations involving emissions of volatile organic compounds at the Garden Grove facility. According to the report, the company agreed to pay $909,935 under the settlement.

Schools within evacuation zones remained closed Monday, while emergency shelters and response operations continued across Orange County as officials worked toward a timeline for residents to safely return home.

Agencies contributed to the story.

renali@chinadailyusa.com