PORT ARTHUR, Texas — Soldiers and police in helicopters and high-water trucks on Thursday rescued thousands more Texans stranded by floodwater from Hurricane Harvey, which has killed dozens of people and displaced more than a million others as it continued to drench the Gulf Coast as a tropical depression.

Also Thursday, chemical maker Arkema SA said it expected more fires after two explosions hit its flooded plant 25 miles northeast of Houston on Thursday, sickening more than a dozen law enforcement personnel and prompting an evacuation of the surrounding area.

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

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 nontoxic, 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.”

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.

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.

Meanwhile, some 779,000 Texans have been ordered to evacuate their homes and another 980,000 fled voluntarily amid concerns that swollen reservoirs and rivers could bring new flooding, according to Department of Homeland Security acting Secretary Elaine Duke.

Jessica Richard, 24, said she waited out the storm in her home in Port Arthur, about 85 miles east of Houston, until Thursday morning when water on her street rose to waist depth. She was picked up by a passing truck.

“All my family is safe even though there were a few close calls,” said Richard, adding that her nephew had been trapped overnight in a flooded apartment with several family members. “He said there were snakes in the water and spiders crawling up the walls. But they got out.”

At least 38 people were dead or feared dead in six counties including and around Houston, according to local officials.

In the U.S. energy hub of Houston, which was paralyzed by the storm earlier in the week, firefighters conducted a block-by-block search of homes to rescue stranded survivors and recover bodies.

In Beaumont, Texas, about 80 miles east of Houston, doctors and nurses evacuated some 190 people from a hospital that halted operations after the storm knocked out water service in the city of almost 120,000 people.

Orange County, which borders Beaumont, on Thursday ordered remaining residents to evacuate the area amid a forecast that the Neches River would crest on Friday, threatening homes.

Just east of Rose City, Texas, a line of cars and trucks was pushing through water flowing over the interstate highway.

A convoy of civilian volunteers from the ad-hoc “Cajun Navy,” who have been carrying out rescues all week, waited as their leader, Phil Drager, negotiated with police to stop traffic and launch an airboat for a scouting mission.

“Now I can add shutting a major interstate highway to my resume,” Drager said.

Vice President Mike Pence visited Texas on Thursday, touring the coastal city of Rockport, where Harvey slammed ashore onto the mainland late last Friday.

“The American people are with you. We are here today, we will be here tomorrow and we will be here every day until this city and this state and this region rebuild bigger and better than ever before,” Pence told a crowd outside a Rockport church damaged by the storm.

President Donald Trump was to return to the region on Saturday.

Moody’s Analytics estimated the economic cost from Harvey for southeastern Texas at $51 billion to $75 billion, ranking it among the costliest storms in American history.

Some signs of normal life returned to Houston.

Residents began a massive cleanup, dragging waterlogged furniture to the curb, hunting for supplies and repair estimates. The city began limited trash pickup and bus services. Hospitals that had struggled to stay open earlier in the week were phasing in clinical operations.

Most of Houston’s larger companies said they would remain closed through the Labor Day holiday on Monday, but employees of smaller businesses and offices were trekking back to work.

Both of the city’s major airports were operating on Thursday and traffic returned to its notoriously clogged highways.

Anita Williams, 52, was in line at a shelter at Houston’s George R. Brown Convention Center to register for aid from the Federal Emergency Management Agency. Williams had returned home Wednesday to survey the damage to her one-story house.

“It’s not my house anymore,” she said. “My deep freezer was in my living room.”

Reuters writer Ben Gruber contributed to this report.