All 50,000 evacuees cleared to return home after chemical tank crisis at Southern California aerospace plant

All evacuation orders stemming from a hazardous materials incident at a Southern California aerospace facility were lifted Tuesday night, allowing thousands of residents to return home.

Officials lifted the final evacuation order for residents in Garden Grove living near a damaged chemical tank at GKN Aerospace Transparency Systems that had previously been at risk of exploding.

Authorities said roughly 50,000 people were ordered to evacuate in and around the Orange County city after a tank containing highly flammable methyl methacrylate (MMA) overheated and became compromised.

"All residents will go home," Orange County Fire Authority Division Chief Craig Covey said during a meeting.

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Over Memorial Day weekend, a crack unexpectedly formed in the tank, relieving pressure and helping avert a catastrophic explosion, allowing most evacuees to return home. Still, roughly 16,000 residents remained under evacuation orders heading into Tuesday.

Crews continued spraying water on the tank until the interior temperature stabilized at 92 degrees, down from 100 degrees over the weekend.

A sprinkler system also doused the tank while company specialists and firefighters removed insulation to help cool it.

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Authorities said evacuation orders were lifted after the tank’s temperature remained stable for four hours without assistance from the sprinkler system.

While health officials have assured residents that monitoring had not detected hazardous levels of contamination or fumes, authorities said they will continue monitoring air quality, sewer systems and storm drains for several months.

Exposure to MMA can cause serious respiratory problems, neurological issues, and irritation to the skin, eyes and throat, according to the Environmental Protection Agency.

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Garden Grove Mayor Stephanie Klopfenstein said she plans to hold the company accountable.

GKN said in a statement Tuesday that it was continuing to work closely with authorities.

"We apologize for the ongoing disruption this incident is causing, and our priority remains the safety of our neighbors and our community," the statement said.

Residents at the meeting questioned why large quantities of the chemical were stored near homes and urged city officials to scrutinize the company’s safety practices.

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Crews had been racing to prevent a catastrophic failure since Thursday, when officials responded to vapor releasing from a in a storage tank at the facility, which manufactures engine structures and products for commercial and military aircraft.

Gov. Gavin Newsom declared a state of emergency Saturday as crews shifted from defensive containment operations to high-risk offensive actions aimed at preventing an explosion.

Orange County Fire Authority Chief TJ McGovern told the Los Angeles Times on Tuesday the crisis was likely caused by the failure of a cooling system designed to regulate the temperature inside the chemical tanks, though officials were still investigating.

He said the malfunction may have caused heat to build inside a pressurized tank containing 7,000 gallons of MMA.

"We don’t know why, but it stopped cooling," McGovern said. "So that’s what started this event, to where the product heated up ... and that’s how this whole response started. We’re just now being able to get to the tanks, so there’s definitely more to come of what caused it."

The Associated Press contributed to this report.