FREMONT, Neb. (AP) — Officials confirmed the deaths of two girls and an adult missing after a massive explosion at a Nebraska biofuels facility on Wednesday. Over a day later, fire teams continued to battle lingering flames, unable to navigate the plant’s precarious wreckage safely.

The children were waiting for a Horizon Biofuels employee to complete a work shift before heading to a doctor’s appointment when the explosion jolted the town around noon on Tuesday, according to Fremont Mayor Joey Spellerberg.

He wasn’t sure of their exact ages but said both were under age 12. Authorities did not provide their names or details on how the three were related.

The facility produces animal bedding and wood pellets for heating and food smoking, using large quantities of wood waste. Spellerberg remarked that officials suspect a wood dust explosion occurred in the plant’s tall elevator tower.

“That’s really the only thing that makes sense,” Spellerberg said. He said Horizon Biofuels is cooperating “as far as I know.”

The company did not immediately respond to phone calls seeking comment.

The explosion severely damaged the top of the elevator tower, revealing a twisted core of concrete and rebar. Below, the building’s metal siding was left buckled and charred, while wisps of smoke lingered in the air on Wednesday, despite overnight rain.

Spellerberg said fire crews were evaluating whether the whole facility might collapse, making it difficult to get inside as they battle the fire.

“It’s going to be very slow,” said Carl Nielsen from the city’s volunteer fire department. No timeline has been set for when they anticipate retrieving the bodies.

“My heart hurts,” Spellerberg told reporters. “It’s a tragedy. We pray for all the families involved.”

The company has 10 employees, according to the Nebraska Manufacturing Extension Partnership.

According to the Fremont Tribune, a 2014 fire had impacted the building’s electrical system but did not compromise its structure. The Occupational Safety and Health Administration notes that significant wood dust accumulation can pose fire and explosion risks.

Taylor Kirklin, who lives about a half mile (0.8 kilometers) from the building, said her whole house shook from the explosion, which was so loud she thought someone had crashed a car into her family’s dog kennel business on the property.

“We were really unsure when the explosion happened which plant it was, because there are so many in that area,” she said. The facility is surrounded by other manufacturing and food processing plants.

Fremont, a city of about 27,000 and the sixth-largest in Nebraska, is 32 miles (52 kilometers) northwest of Omaha.

Emily Anderson, who lives just blocks from the plant, said she heard “one really big boom” before police cars flooded in.

“There were just huge plumes of very, very black smoke,” Anderson said. “It was scary.”