Arkema, the French chemicals group that runs the plant, said the Harris County Emergency Operations Center notified company officials early Thursday of two explosions and black smoke coming from the facility, which is about 25 miles northeast of Houston. The plant already had been under about six feet of water from the rains unleashed by Harvey, prompting officials the day before to establish a 1.5-mile evacuation zone.

Officials later downplayed the severity of what happened at the plant, which makes organic peroxides for use in items such as counter tops and pipes. Those materials are currently being stored inside nine separate, 18-wheeler box vans at the facility, weighing 36,000 pounds each.

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Authorities said Thursday morning that there weren’t “explosions” at the facility but, rather, “small pops” followed by smoke and fire.

But Richard Rennard, an Arkema executive, said it was impossible to know for sure, since all the employees had left the site.

“These things can burn very quickly and violently; it would not be unusual for them to explode,” Rennard said at a news conference. However, he said: “We believe it hasn’t been a massive explosion; it’s just been these vapor release valves that popped” in one of the box vans.

“We fully expect that the eight other containers will do the same thing,” Rennard added. “We anticipate that all of this product is going to degrade; we don’t know exactly how long that’s going to take … whether it’s today, tomorrow, we just don’t know; it’s impossible to predict.”

The Harris County Fire Marshal’s Office reported “a series of chemical reactions” and “intermittent smoke” at the facility sometime between midnight and 1 a.m. Thursday.

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Bob Royall, assistant chief for emergency operations for the Harris County Fire Marshal’s Office, said there were “small container ruptures that may have a sound” — like “a series of pops.”

“I don’t want the public thinking these are massive explosions,” Royall told reporters, adding: “I call it a chemical reaction and an overpressure of the container.”

Harris County Sheriff Ed Gonzalez told reporters that the fumes created by the chemical reaction are “not anything toxic; it is not anything that we feel is a danger to the community at all.”

Federal authorities at first used dire language to describe the impact of those fumes: A potential chemical plume in Crosby could be “incredibly dangerous,” William “Brock” Long, administrator of the Federal Emergency Management Agency, said at a briefing Thursday morning. An earlier study done for the Environmental Protection Agency found that organic peroxides are skin and eye irritants and could also cause liver damage.

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FEMA later clarified Long’s comments, saying that overflights by chemical-detecting aircraft showed the smoke did not contain “concerning” levels of hazardous materials. The agency added that it would defer to local officials to make warnings and safety determinations, the Associated Press reported.

Fifteen deputies were evaluated by medical teams as a precaution, and all of them have been released from a hospital after experiencing what Gonzalez, the sheriff, referred to as respiratory irritation.

“We believe the smoke is a nontoxic irritant,” Gonzalez said.

Rennard, the Arkema executive, encouraged anyone exposed to the smoke “to call their doctor or seek medical advice.”

Authorities and Arkema officials had warned earlier that such problems were likely.

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The material at the plant must remain cold — otherwise it can combust. “The material naturally degrades, and some types can be unstable unless refrigerated,” Arkema explained.

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The facility’s coolant system and inundated backup power generators failed Tuesday, according to the company. Primary power at the plant went out Sunday, and two sources of emergency backup power were lost shortly thereafter.

At that point, Richard Rowe, chief executive of Arkema’s North American unit, warned that trouble was likely.

“We have lost critical refrigeration of the materials on site that could now explode and cause a subsequent intense fire,” Rowe said in a statement Wednesday. “The high water and lack of power leave us with no way to prevent it. We have evacuated our personnel for their own safety. The federal, state and local authorities were contacted a few days ago, and we are working very closely with them to manage this matter. They have ordered the surrounding community to be evacuated, too.”

In the statement, Rowe apologized “to everyone impacted by our situation.”

A mandatory evacuation zone was established for a 1.5-mile radius Wednesday as the last remaining workers at the facility attempted to resolve the problem. Police cruisers and SUVs sealed off access to the plant on Highway 90, which connects Houston and Beaumont. Parts of the highway nearby were underwater.

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A continuous flow of trucks, many hauling boats to participate in flood rescue efforts, approached the police barricade near the facility Wednesday afternoon only to be turned away as Crosby Volunteer Fire Department trucks crisscrossed the highway cut-through roads.

On Thursday, as authorities addressed the issues in and around the Arkema plant, Crosby residents forced to move outside the evacuation zone tried to carry on, anxious to return to their homes.

Families packed a local McDonald’s, ordering breakfast sandwiches and watching their children run around the restaurant’s play area — even as the remnants of ammonia vapors drifted over Crosby. An abrupt loss of cellphone service frustrated many of them, who tried using the McDonald’s WiFi to stay updated. (An AT&T spokesman told The Washington Post that the network came down as a result of the Arkema chemical plant incident but provided no further details.)

Crosby resident Kenneth Mayfield’s mind was on his animals: horses, a dog and a pig.

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He was most concerned about Champion, his prized stud, which was about half a mile from ground zero of the Arkema chemical plant.

“That’s exactly what he is: a champion,” Mayfield said at the McDonald’s off Highway 90, about five miles from the plant and near where police sealed off the highway approach.

After the mandatory evacuation was ordered Wednesday, Mayfield and his wife, Rodneisha Cheatham, could not make it back to the makeshift ranch where Champion was kept. They had been forced to evacuate because of the plant’s danger, not the flooding, Mayfield said.

His twin Texas-outline tattoos — illustrating “P” and “A” near both his elbows — showed hometown pride for nearby Port Arthur. He was also eager to hear updates on the safety of relatives there, in Orange and Beaumont.

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“I have family all through the worst places of the flood,” Mayfield said.

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Frustrated, Mayfield placed his elbows on the table in a window-side booth to think about a crafty route to his horses.

“I know a route through the woods,” he said, “but I don’t want to get into that water.”

David Guillory, who also lives in Crosby, said he had the same problem.

Guillory held a hand at his waist. “About this high,” he said, of the water level in his house about a half-mile from the plant.

With the skies over rural Crosby opened to the bright sun Thursday, Guillory and others in town were hoping to get to hardware stores to start repairing damage. However, road closures related to the plant incident had shut down access in every direction.

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Guillory said he was skeptical about efforts to notify people living near the plant, which includes some of worst flooding in the area. He wondered if there were people trapped or who had not yet evacuated because of road closures who might not know about the danger from the Arkema facility.

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Local police told him everyone was safely evacuated, but his brother, who lives right on the edge of the evacuation zone, was still home when Guillory called Wednesday.

Guillory said his water-deluged home is much closer to the plant.

“It’s in my backyard. Literally,” he said.

The Arkema facility, the company noted, “is in a rural area with no hospitals, schools, correctional facilities or recreational areas or industrial/commercial areas in the vicinity.” Arkema said the plant, which employs 57 people, “has never experienced flooding of this magnitude before.”

Ahead of Harvey’s arrival, “the plant made extensive preparations,” bringing extra backup generators to the facility, along with diesel-powered refrigerated tank trailers, Arkema said. But the generators were inundated by water and failed. At that point, the company said, “temperature-sensitive products” were transferred into the diesel-powered refrigerated containers.

Still, the company said Wednesday, “the most likely outcome is that, anytime between now and the next few days, the low-temperature peroxide in unrefrigerated trailers will degrade and catch fire. There is a small possibility that the organic peroxide will release into the floodwaters but will not ignite and burn. … In the alternate, there could be a combination event involving fire and environmental release. Any fire will probably resemble a large gasoline fire. The fire will be explosive and intense. Smoke will be released into the atmosphere and dissipate. People should remain clear of the area.”

The Associated Press reported that Arkema was previously required “to develop and submit a risk management plan to the Environmental Protection Agency, because it has large amounts of sulfur dioxide, a toxic chemical, and methylpropene, a flammable gas.”

The plans are supposed to detail the effects of a potential release, evaluate worst-case scenarios and explain a company’s response. In its most recently available submission from 2014, Arkema said potentially 1.1 million residents could be impacted over a distance of 23 miles (37 kilometers) in a worse case scenario, according to information compiled by a nonprofit group and posted on a website hosted by the Houston Chronicle. But, Arkema added, it was using “multiple layers of preventive and mitigation measures” at the plant, including steps to reduce the amount of substances released, and that made the worst case “very unlikely.”

Water will dilute the chemicals in the plant but also make them difficult to contain; just as the plant was unable to keep water from flowing in, it will have trouble controlling water flowing out.

An industry safety guide notes that fire or explosion will release a variety of chemicals, including carbon dioxide, as well as flammable vapors including methane or acetone. This could accelerate the decomposition of the chemicals.

The guide said that water is “usually the agent of choice to fight fire,” though warm water could accelerate the breakdown, and ignition, of the organic peroxides.

In February, Arkema’s Crosby plant was initially fined $107,918 for 10 violations by the Occupational Safety and Health Administration, federal records show. The violations were marked as “serious,” meaning they were hazards that could cause serious physical injury or death 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.

Wang and Bever reported from Washington. Steven Mufson, Jack Gillum, Brian Murphy, Mark Berman and J. Freedom du Lac contributed to this report, which has been updated.