(LOS ANGELES) -- The evacuation zone around a failing chemical tank in Southern California is shrinking after officials said the "worst-case scenario" of a catastrophic explosion has been averted.

About 16,000 people remain under evacuation orders, Garden Grove Police Chief Amir El-Farra said during a press briefing Monday evening. That's down from 50,000 in the city of Garden Grove and several surrounding communities at the height of the crisis when officials thought they faced only two options: an explosion or a chemical spill.

"The most catastrophic and worst-case scenario was mitigated and resolved," Orange County Fire Authority (OCFA) Interim Chief TJ McGovern said during the evening press briefing.

Officials said earlier Monday the crisis is not fully averted, but if an explosion or leak were to occur, it would be significantly smaller than the initial worst-case scenario.

The tank is located at an aerospace facility in Garden Grove, about 30 miles south of Los Angeles.

Earlier Monday, local authorities said the temperature was declining within the chemical tank, located at an aerospace facility, eliminating concerns of an explosion, local authorities said on Monday morning.

"We are happy to report that the threat of a BLEVE [Boiling Liquid Expanding Vapor Explosion] is now off the table," McGovern said during an update. "That threat has been eliminated."

Since the temperature inside the tank has decreased, it has allowed the chemical inside to solidify, but it is unclear how much, according to McGovern.

The situation began unfolding on Thursday, a chemical tank filled with toxic chemicals at GKN Aerospace, a manufacturing company that builds engines and landing gear for both commercial and military aircraft, was showing signs of overheating, which could cause it to overheat or spill, officials said.

The 34,000-gallon tank contained methyl methacrylate, an industrial chemical used in plastic manufacturing, according to the OCFA. The chemical is primarily a respiratory irritant. Short-term exposure can cause skin and eye irritation, as well as breathing problems, according to the EPA.

The "unprecedented" situation caused officials to order about 50,000 people in the vicinity to evacuate in case a leak sent toxic fumes wafting through the neighborhood or caused an explosion could result in a dangerous fireball.

California Gov. Gavin Newsom issued a state of emergency in Orange County in response to the incident.

Authorities said a crack in the tank, discovered by firefighters late Saturday, relieved some of the pressure within the tank, Covey said.

While officials were confident that the crack wouldn't lead to any chemical leaks, they continued to monitor air quality in the region.

Any areas outside of the roughly 10-square-mile evacuation zone "are currently considered completely safe and day-to-day activities can continue as normal," the OCFA said in an update Sunday afternoon.

Within the zone are schools, hospitals, nursing homes, fire and law enforcement stations and critical infrastructure. Multiple roads were also closed in the area.

"We appreciate your support and the patience while we work through this incident together," Covey said.

In a statement on Monday, GKN noted that its technical specialists worked with OCFA "to assess the storage tank more closely" on Sunday evening.

"The team safely and successfully removed external insulation material from the tank in order to help advance efforts to cool its contents," the company said.

Orange County District Attorney Todd Spitzer, the top prosecutor in Garden Grove, has launched a criminal probe into the ongoing chemical tank incident at GKN Aerospace, ordering the company not to destroy or manipulate any records, his office said.

The probe seeks to determine how a major military and commercial aircraft gear manufacturer could have allowed such a toxic failure to occur, according to the district attorney.

"This is an incredibly volatile situation with extraordinary efforts being made by first responders to prevent a potentially catastrophic disaster," Spitzer said in a statement on Monday.

Spitzer has also established an anonymous tip line and online reporting form, calling on anyone with information to come forward -- including current and former employees of the company who might offer insight into the quality control and safety at the site.

"Given the very real risk to human life as a result of this event, it is crucial that anyone who has information about this incident or the industrial operations of GKN Aerospace Transparency Systems, Inc. come forward and report it so that it can be thoroughly investigated by law enforcement," he said.

GKN declined to comment specifically on the district attorney's investigation, but pointed to its Monday statement.

"We are continuing to work around the clock with the OCFA, the EPA and all relevant federal, state and local agencies to mitigate the ongoing risk of a leak. We remain extremely thankful for their dedication and hard work," the company said.

"We apologize for the ongoing disruption this incident is causing and our priority remains its safe resolution, so that residents can return to their homes as quickly as possible."

ABC News' Meredith Deliso, Nadine El-Bawab, Jaclyn Lee, Alex Stone, Jenna Harrison, Connor Burton and Sasha Pezenik contributed to this report.

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