HOUSTON (AP) — Thick black smoke and towering orange flames shot up Friday after two trailers of highly unstable compounds blew up at a flooded Houston-area chemical plant, the second fire there in two days.

Arkema says Harvey’s floodwaters engulfed its backup generators at the plant in Crosby and knocked out the refrigeration necessary to keep the organic peroxides, used in such products as plastics and paints, from degrading and catching fire. Arkema executive Richard Rennard said two containers caught fire Friday evening, and that there are six more it expects will eventually catch fire.

Arkema spokeswoman Janet Smith said that the company expects the rest of the containers will ignite “within a matter of days.”

Preliminary analysis of data captured by Environmental Protection Agency surveillance aircraft Friday did not show high levels of toxic airborne chemicals, agency spokesman David Gray said. No serious injuries were reported in the last two days as a result of the fires.

The height and color of the flames from the plant Friday suggested incomplete combustion of the organic peroxides, Texas A&M chemical safety expert Sam Mannan said. With complete combustion, he said, the byproduct is carbon dioxide and water, posing about the same amount of risk as standing too close to a campfire. But incomplete combustion implies something else is burning.

The fire burned not just the organic peroxides but also the plastic packaging, insulation, and the materials used to construct the trailers, Smith said.

Daryl Roberts, the company’s vice president of manufacturing, technology and regulatory services in the Americas, told The Associated Press on Wednesday that the floodwater inundating the plant would cause any toxins produced by the fire to quickly vaporize. By Friday, the water had receded but Smith could not comment on whether that had changed the calculation of risk.

A 1½-mile buffer (2.4 kilometers) around the plant was established Tuesday when Arkema warned that chemicals kept there could explode. Employees had been pulled, and up to 5,000 people living nearby were warned to evacuate. Officials remain comfortable with the size of the buffer, Rachel Moreno, a spokeswoman for the Harris County Fire Marshal Office, said Friday evening.

Smith reiterated statements executives made earlier Friday that the safest course of action was simply to “let these fires happen and let them burn out.”

Arkema officials did not directly notify local emergency managers of the generator failure, Moreno said. It instead came by way of the plant’s ride-out crew, who told the Crosby Volunteer Fire Department about it when they were rescued during the storm, she said.

In a conference call with reporters Friday, Arkema President and CEO Rich Rowe apologized and said he was sending a team of employees to Crosby to figure out how best to assist locals.

“I realize this is not a situation that we can help remedy overnight,” he said.

Early Thursday, two blasts blew open a trailer containing at least 2 tons of material, sending up a plume of black smoke and flames 30 to 40 feet (9 to 12 meters) high in the tiny town of Crosby, about a half-hour from Houston, authorities said. The Texas environmental agency called the smoke “especially acrid and irritating” and said it can impair breathing and inflame the eyes, nose and throat.

Questions persisted Friday about the adequacy of Arkema’s master plan to protect the public in the event of an emergency in flood-prone Houston, a metropolitan area of about 6 million people.

The plant is along a corridor with one of the nation’s greatest concentrations of refineries, pipelines and chemical plants. A 2016 analysis led by Texas A&M University researchers identified Arkema’s facility as posing one of the region’s biggest risks, based on such factors as the type and amount of chemicals and the population density.

In accident plans Arkema submitted to the EPA in 2014, executives said a hurricane and a power loss were potential hazards. Yet the plans, which were supposed to address worst-case scenarios, didn’t explain what Arkema would do if faced with either.

Executives also acknowledged Friday that they didn’t have materials at the plant that could have neutralized the organic peroxides.

Instead, workers were forced to scramble and move the chemicals away from floodwaters after buildings were engulfed and power was lost. Workers transferred the compounds to refrigerated containers, but those failed, too, causing Thursday’s fire.

After days of questions about what was in its chemical inventory, the company posted a list of them on its website Friday, though not the amounts on hand. Asked why it hadn’t shared the information sooner, Rowe said, “We’re managing our way through a crisis.”

State and federal regulators have cited Arkema for safety and environmental violations at the Crosby plant dating back more than a decade, records show.

Texas’ environmental commission penalized the plant at least three times. In June 2006, Arkema had failed to prevent unauthorized emissions during a two-hour warehouse fire. Records show a pallet of organic peroxide was poorly stored, resulting in the blaze, and more than a ton of volatile organic compounds were discharged.

More recently, the U.S. Occupational Safety and Health Administration in February fined Arkema nearly $110,000 — later reduced to just over $90,000 — because of 10 serious safety violations found during an inspection.

“We don’t have a perfect record, we understand that,” Rowe said. “We strive to get better at every turn and will continue to do so.”

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Dunklin reported from Dallas. Associated Press writers Michael Biesecker, Matthew Daly and Seth Borenstein in Washington; Luke Sheridan in New York; and Angeliki Kastanis in Los Angeles contributed to this report.