All remaining evacuation orders were lifted in Orange County on Tuesday evening as authorities declared there was no remaining danger of an explosion, chemical leak or fire stemming from a hazardous material incident at an aerospace manufacturing plant.

Around 50,000 people in six Orange County cities were told to evacuate during the crisis, when an overheating tank filled with 7,000 gallons of a chemical called methyl methacrylate — a highly flammable liquid monomer used to manufacture plastics — was at risk of causing a massive explosion.

According to a recent report from the California Governor’s Office of Emergency Services, the sudden heating of the tank caused the storage tank to bulge, and fumes were released for about five minutes before employees at the company activated a manual fire-suppression system.

Fire officials initially offered the dire assessment that the unstable tank would either explode or fail, causing a massive leak. But as families were told they could return to their homes, neither of those options had come to pass.

At 7:30 p.m. Tuesday, the Orange County Fire Authority announced that the decision to fully repopulate the evacuation zone was made in coordination with law enforcement, the Environmental Protection Agency and the OC Health Care Agency.

Evaluations over Memorial Day weekend found that a crack in the tank at GKN Aerospace in Garden Grove had reduced the pressure inside, drastically lowering the risk of an explosion, according to interim Orange County Fire Authority Chief TJ McGovern. Crews were then able to peel back the external wall and insulation and closely target their water hoses to continue cooling the chemical inside, McGovern said.

Officials had announced Monday morning that the risk of a BLEVE, or boiling liquid expanding vapor explosion, had been eliminated after crews sprayed more than 9 million gallons of water onto the tank.

As a result, the evacuation zone was initially reduced Monday evening, allowing some 34,000 residents to return home.

On Tuesday, crews remained focused on stabilizing the temperature of the tank to eliminate any remaining fire risk and on monitoring runoff to ensure sure no contaminated water escaped.

The tank was sitting at 92 degrees on Monday morning, according to Fire Authority spokesperson Greg Barta. Crews then turned off the water stream and closely assessed the response.

Craig Covey, division chief with the Orange County Fire Authority, interrupted a community meeting in Garden Grove on Tuesday evening to break the news to residents that all evacuations were being lifted.

“I just got positive news that the tank [temperature] stayed stable since the three or four hours that we turned the water off,” he said. “I did not want to delay you getting home tonight. I’m sorry to do this right in the middle of this meeting, but this is more important than anything else.”

Out of an “abundance of caution,” a 300-foot exclusion zone, which does not affect any residents, remains in place around the tanks as hazardous materials crews continue monitoring conditions. A road closure is also in effect along Western Avenue between Chapman and Garden Grove boulevards.

During a news conference Wednesday morning, OCFA Battalion Chief Mike Yeun said crews would remain on scene to monitor the state of the methyl methacrylate and that, despite the mitigated risk, it was unclear how long crews would stay.

“What we don’t want is an increase in temperature,” Yeun said.

Yeun said residents should have no concerns in returning back home, and that crews would remain until the chemical stabilized to a point where they could move into a “removal phase.”

According to the Governor’s Office of Emergency Services, about 5,000 to 7,000 gallons of the chemical still remains inside the tank in liquid form.

Chris Myers, an EPA coordinator, said Tuesday that the agency has been monitoring the air quality around the area as well, with “no exceedances detected.”

But as thousands of families began returning home Tuesday evening, questions still remained about the crisis that upended six cities and caused a near-catastrophe.

The Orange County district attorney’s office has opened an inquiry into the cause of the emergency, as well as operations at the aerospace company over the weekend, and set up an anonymous tip line for information.

“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,” Dist. Atty. Todd Spitzer said in a statement.

Gov. Gavin Newsom proclaimed a state of emergency for Orange County on Friday and, on Monday, President Trump approved the state’s request for a presidential emergency declaration, allowing the Federal Emergency Management Agency to provide federal financial assistance.

On Wednesday, the Orange County Board of Supervisors was set to ratify their own emergency proclamation, which would make money available to residents, businesses and the county from sources such as the California Disaster Assistance Act fund and State Private Nonprofit Organizations Assistance Program.

According to the county’s staff report, the full scope of the damage, and the cost of responding to the crisis, is still being tallied.

The six-day chemical crisis began inside the GKN Aerospace facility in Garden Grove and was probably caused by the failure of a cooling system designed to regulate the temperature of chemical tanks, McGovern said.

Multiple lawsuits have already been filed against the aerospace manufacturing company seeking compensation for residents exposed to danger and forced to evacuate.

The Fire Authority initially responded to the incident Thursday afternoon by dousing the tank in water and issuing an evacuation order for surrounding streets that was lifted that evening.

But by Friday morning, the severity of the crisis became apparent as crews were unable to neutralize and remove the chemical.

The tank was deemed at risk of causing a catastrophic explosion and sweeping evacuation orders were issued affecting residents in areas of Garden Grove, Cypress, Stanton, Anaheim, Buena Park and Westminster.

The Orange County health officer and fire officials have insisted there have been no toxic chemicals released over the course of the emergency.

“There was no contamination, there were no fumes, there were no vapors that came from this incident,” said Dr. Regina Chinsio-Kwong, county health officer, said Monday. “There was no leak.”

Some environmental experts have expressed skepticism over this claim, arguing that it is likely at least some vapors escaped from the crack in the tank.

Times staff writers Hannah Fry and Tony Briscoe contributed to this report.