When the hurricane blew in, workers at the Arkema chemical plant in Crosby, Tex., faced the problem of keeping the plant’s volatile chemicals cold. The plant had 19.5 tons of organic peroxides of various strengths, all of them requiring refrigeration to prevent ignition.

But the power went out, and then the floodwaters came and knocked out the plant’s generators. A liquid nitrogen system faltered. In a last-ditch move, the workers transferred the chemicals to nine huge refrigerated trucks, each with its own generator, and moved the vehicles to a remote section of the plant.

That was doomed to fail, too. Six feet of water swamped the trucks, and the final 11 workers gave up. At 2 a.m. Tuesday, they called for a water evacuation and left the plant to its fate.

Early Thursday, two loud pops signaled an explosive combustion in one of the trucks, and a black plume of smoke spread from the plant, sending 15 police officers and paramedics to the hospital. All eight remaining vehicles are now likely to burn, said Robert W. Royall Jr., assistant chief of emergency operations for the Harris County Fire Marshal’s Office.

We are “watching physics at work,” Arkema spokesman Jeff Carr said Thursday. “Probably a couple more tonight.”

Explosion evacuee Martha Higdon and her son Truman, left, speaks outside of the First Baptist Church, which has been set up as a shelter for residents evacuated from their homes following an explosion at the Arkema Chemical Plant in Crosby, Texas. (Mandel Ngan/AFP/Getty Images)

Daryl Roberts, Arkema’s vice president of manufacturing, told reporters Friday that “the water has begun to recede at the site.”

But he said that even if more parts of the site become accessible in coming days, company officials does not believe it will give them the ability to restart refrigeration. For starters, the electrical infrastructure on the site has been underwater for more than a week, he said, he will likely need extensive work.

“We’re not in a position to quickly establish cooling,” he said.

He said the company also does not want to put its employees or emergency officials in harm’s way when the containers of chemicals could ignite at any time.

“We believe that right now, the scenario that is available to us is to let that material burn out,” Roberts said

While this crisis has not yet equaled the severity of explosions at other Texas chemical plants, the crisis at Crosby has exposed the vulnerability of hundreds of chemical plants in low-lying areas across the U.S. Gulf Coast.

“The Crosby plant’s dangerous situation is a symptom of a bigger problem involving the oil and chemical industry in the gulf region,” said Bill Hoyle, a former senior investigator for the Chemical Safety Board and now an independent safety consultant. “The Crosby plant is a wake-up call for an industry and their safety regulators who have not adequately taken action on lessons from Hurricane Katrina as well as the Fukushima nuclear disaster in Japan.”

Texas has more than 1,300 chemical plants, a large number of them in low-lying areas near the coast that are vulnerable to flooding. Arkema’s Crosby plant was built decades ago, but access to gulf ports and the surge in shale gas operations in Texas and Louisiana have lured scores of new chemical plants to the Gulf Coast region.

Although the fire and blasts have so far not been as dire as many feared, the loss of control of dangerous materials and the igniting of volatile chemicals spread anxiety and triggered an investigation by the Chemical Safety Board, an independent federal agency.

The plant produced organic peroxides, which are used in a variety of products including pipes, plastics, acrylic paints, countertops and pharmaceuticals. A company spokesman estimated that 19.5 tons of chemicals were at the site. Small amounts can irritate the skin or damage corneas, and in larger amounts could cause liver damage, according to the Occupational Safety and Health Administration (OSHA). But the company spokesman said “the issue is a combustion event, not a chemical release.”

The Arkema emergency raises anew a host of concerns for chemical manufacturers. After the 1984 tragedy in Bhopal, India, in which a chemical leak from a Union Carbide plant killed more than 2,000 people and injured many thousands more, then-Sen. Frank R. Lautenberg (D-N.J.) pressed for legislation requiring chemical companies to describe their own worst-case scenarios.

Arkema, whose slogan is “Innovative Chemistry,” filed one of those reports in June 2014 for its plant in Crosby, warning that in the most catastrophic scenario, 1.1 million people within a 23-mile radius would be affected. In Texas alone, 32 other plants also warned that more than a million people could be affected by a chemical catastrophe, according to a Congressional Research Service report.

But Arkema stressed that “multiple layers of preventive and mitigation measures in use at the Crosby facility make it very unlikely” that a worst-case scenario would occur. And “in the unlikely event that such a release occurs, Arkema, Inc. has mitigation measures in place to reduce any potential impacts.”

This week, however, some layers of preventive measures failed.

“Certainly, we didn’t anticipate having six feet of water in our plant,” Richard Rennard, president of Arkema’s acrylic monomers division, told reporters Thursday.

Hundreds of plants have been shut down since Hurricane Harvey approached Texas last week, posing environmental dangers as they restart their waterlogged facilities.

About 5 percent of Texas facilities registered in the EPA’s Toxics Release Inventory Program were plotted in or adjacent to flooded areas observed from satellite imagery through Wednesday, according to a Washington Post analysis. They included factories that produce petroleum, plastics and rubber, and deal with hazardous waste. Of those, 23 deal specifically with chemicals.

Arkema, a spinoff of the French oil giant Total, has more than 30 sites in the United States, and like other operators in the industry, has lobbied federal regulators to delay new regulations designed to improve safety and disclosure at chemical plants.

The company has also run afoul of OSHA regulations.

In February, Arkema’s Crosby plant was initially fined $107,918 for 10 OSHA violations, federal records show. The violations were marked as “serious,” meaning they could cause serious physical injury or worker deaths if not remedied. One included a violation of inspection procedures that were supposed to “follow recognized and generally accepted good engineering practices.”

The government later reduced the fines to about $91,000.

Arkema also agreed to a settlement with the Texas Commission on Environmental Quality (TCEQ) in January stemming from a leak of a toxic and flammable compound in June 2016, state records show. The plant released 4,800 pounds of isoamylene after workers left a valve partially open for 62 hours, allowing the chemical to drain from a storage tank, according to enforcement records.

A state inspection of the facility months earlier also found seven violations. The TCEQ lists the company’s overall compliance history as “satisfactory,” however. For the June leak, the commission imposed a modest fine after concluding that residents and the environment had been exposed to “insignificant amounts” of pollutants.

Even in the current crisis at Crosby, Royall, the Harris County emergency operations official, said that the danger from the Arkema plant was “really relative.”

“If you’re standing right next to something and you had a chemical release, it would probably be pretty dangerous, I think you’d agree,” Royall said. “But we have a mile-and-a-half safety radius, and there’s nobody in that plant.”

The events at the plant cause more worries for residents already dealing with inundated homes. But for some residents, the threat is not extraordinary.

There have been so many plant explosions in the Houston area that resident Robin Boethin cannot keep them straight. She recalled the Texas City refinery explosion in March 2005 — not to be confused with the Texas City disaster of 1947, one of the deadliest industrial accidents in U.S. history. Then there was the Pasadena incident in October 1989, in which gases ignited a series of explosions, killing 23 workers and injuring 300.

“It was a ka-boom type of thing,” she said from the counter of the Rusty Bucket, her antiques shop in Crosby, a few miles from the chemical plant. “It shook the house so bad I called 911. I thought someone was breaking in.”

Boethin and others in Crosby discussed chemical plant explosions and environmental disasters as a way of life in the Houston area, describing the risk of sprawling chemical sites as Californians might discuss the inevitability of the next earthquake.

“There’s danger and everyone knows it,” she said.

In the emergency response plan filed with the EPA in 2014, Arkema sketched out the possible disaster that would follow from the failure of one of its tanks of 2-methylpropene. It wouldn’t exactly be a fire or an explosion, but a fiery combination known in the chemical industry as a “bleve,” short for “boiling liquid expanding vapor explosion.”

In that grave scenario, the sudden release of flammable, toxic vapor could ignite in a fireball with a lethal “thermal radiation dose” that could extend over 1,000 feet — “approaching the yard of the residence nearest to the site.”

At a news conference Thursday, Arkema’s Rennard repeatedly and evenly walked reporters through the steps taken at the plant and the outlook for the coming days.

“We anticipate that all this product is going to degrade,” he said. “Whether it’s today, tomorrow, we just don’t know. It’s impossible to predict that.”

One reporter shouted, “Do you understand people are worried?”

“Of course we understand that,” Rennard said, “and that’s why we want to make sure people respect this one-and-a-half-mile radius. We don’t want people returning back to their homes thinking it’s over. It’s not over.”

Jack Gillum, Aaron C. Davis, Julie Tate, Andrew Ba Tran and Alex Horton contributed to this report. Horton reported from Crosby, Tex.