CROSBY, Texas (Reuters) – Chemical maker Arkema SA (AKE.PA) said it expected more fires after two explosions hit its flooded plant 25 miles (40 km)northeast of Houston on Thursday, sickening more than a dozen law enforcement personnel and prompting an evacuation of the surrounding area.

Meanwhile, the U.S. Chemical Safety Board said on Thursday afternoon it had launched an investigation into the explosions and fires at the 49-year-old chemical plant in Crosby, Texas.

In a press call with reporters and a statement on Facebook, board Chairwoman Vanessa Allen Sutherland said the independent federal agency would begin its probe with requests for documents from the chemical maker and would not deploy to the site of the explosions until emergency response activities were completed and the facility was deemed safe.

Arkema, a French company, said all four of its systems to cool the organic peroxides produced at its Crosby, Texas, plant and stored onsite in refrigerated containers were expected to fail, triggering their degradation and eventually more fires.

Arkema and local officials said they believed the smoke from the blaze was non-toxic, but they urged people to stay away as the fire burns itself out,

“Any smoke is going to be an irritant,” Richard Rennard, the head of one of Arkema’s business units, told reporters near the scene. “Certainly, these things burn when they degrade, and there is the possibility that an explosion could happen.”

Shares of Arkema fell 1.9 percent in Paris trading.

Before Sutherland announced the investigation, two Democratic U.S. senators, Tom Carper of Delaware and Cory Booker of New Jersey, sent a letter to her seeking such a probe.

Rennard and Crosby Assistant Fire Chief Bob Rayall both said pops heard at the scene were the sounds of container pressure valves failing as force from the warming chemical built up.

One of nine containers with the organic peroxides had caught fire, and Rennard said Arkema expected the remaining eight to burn eventually. Company officials do not expect to have access to the site for up to five more days because of high water levels.

A joint statement from the federal Environmental Protection Agency as well as local and state officials issued Thursday afternoon said, “At this time, we are responding to a fire, not a chemical release.

“We continue to monitor smoke and air quality. … As with all smoke, people can limit the potential for adverse health effects by limiting their exposure.”

The peroxides, which are used to make plastic resins, polystyrene, paints and other products, are extremely flammable if they are not kept at low temperatures.

The EPA said it had emergency personnel on the scene, and initial indications are “no concentrations of concern for toxic materials.” The EPA is sending air-monitoring personnel to the site.

The Federal Aviation Administration said Wednesday it had temporarily barred flights from the area because of the risk of fire or explosion.

Arkema said on Wednesday it had no way to prevent fires because the plant has about 6 feet (1.83 meters) of water due to flooding from Harvey. The hurricane came ashore in Texas last week, knocking out power to the plant’s cooling system.

The company said the Harris County Emergency Operations Center notified it at about 2 a.m. (0700 GMT) of two explosions and black smoke coming from the plant.

Harris County Sheriff Ed Gonzalez told reporters 15 deputies were taken to a hospital after concerns some may have inhaled fumes. All have since been released, his office said later on Twitter.

The plant had been closed since Friday, and the company evacuated remaining workers on Tuesday. Harris County ordered the evacuation of several hundred residents within a 1.5-mile (2.4 km-) radius on Tuesday.

“We want local residents to be aware that product is stored in multiple locations on the site, and a threat of additional explosion remains,” Arkema said. “Please do not return to the area within the evacuation zone.”

In a risk management plan filed with the EPA in 2014, Arkema said a worse-case scenario at the plant could threaten people as far as 23 miles away, affecting a population of more than 1 million. The next such report is not due until 2019 for the plant.

Such a scenario would entail the complete rupture of its isobutylene or sulfur dioxide feedstock tanks and the failure of all backup safety systems. On Thursday, Arkema said it did not expect the fire to extend beyond the organic peroxide tanks.

An Arkema official did not immediately respond to a question about whether the current evacuation area could be expanded.

Harris County Emergency Management referred questions on the Arkema risk management plan to local and county fire officials, who did not immediately respond to phone calls.

One resident pleaded with police to check on an elderly couple, Leo and LeJane Opelia, who she said live two streets from the plant and returned home on Wednesday night to look in on their cats and belongings after leaving earlier.

“I heard from them last night, and now I keep trying to call them, and I can’t reach them,” said a tearful Frances Breaux. “I just want to make sure that they are OK.”

She said police told her the area was too dangerous to enter.