The remaining 16,000 residents of a Southern California city were allowed to return home Tuesday, days after the threat of a catastrophic explosion at a chemical tank prompted fears of an environmental disaster.

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The crisis at the 7,000-gallon tank of the toxic and flammable chemical methyl methacrylate at GKN Aerospace in Garden Grove, first detected Thursday, had prompted the evacuation of tens of thousands of people.

By Tuesday night, there was no longer a risk of explosion, no chemical leak, no threat of fire and no risk to the public, the Orange County Fire Authority said.

“All evacuation orders related to the Garden Grove hazardous materials incident have been lifted,” the agency said.

At its peak, the emergency had forced around 60,000 people in Garden Grove and neighboring cities from their homes. Most evacuation orders were lifted Monday, but around 16,000 people had been told to continue staying away because there remained a small risk of a fire or explosion.

Some residents at a community meeting Tuesday questioned why dangerous chemicals were allowed at the facility in Garden Grove, a city of around 171,000 residents 35 miles southeast of Los Angeles.

Mayor Stephanie Klopfenstein said there will be investigations at the city, state and federal levels.

“Our community deserves to know what happened, why it happened and whether laws or regulations were violated,” she said.

Firefighters responded to a 911 call at 3:22 p.m. Thursday about an active leak at GKN Aerospace, Orange County incident commander Craig Covey said at the meeting, and found a pressure valve at the top of one of three tanks releasing vapor.

A problem with a valve in the refrigeration system, which was supposed to keep the tank at 50 degrees Fahrenheit, had caused overheating, he said.

“That tank was imminently ready to blow,” Covey said.

Sprinklers from the company were on and trying to cool the tank, he said, and the fire department used a hose stream pouring 1,250 gallons a minute on it.

“Those cooling measures were successful; it reduced the pressure,” Covey said.

A gauge showed the tank had reached at least 100 degrees over the weekend, Covey said. On Tuesday, the temperature at the main tank was holding at 92 degrees, the fire agency said.

A crack on the tank also helped relieve pressure and allowed crews to take more aggressive measures. Had the worst fear — a boiling liquid expanding vapor explosion, or BLEVE — occurred, it would have been a catastrophe, officials have said.

A spokesperson for GKN said earlier Tuesday that “we are continuing to work around the clock” with the Orange County Fire Authority, the Environmental Protection Agency and others.

“We apologize for the ongoing disruption this incident is causing, and our priority remains the safety of our neighbors and our community,” the spokesperson said in a statement.