FREMONT, Neb. (AP) — Two girls and an adult missing since a huge explosion tore through a Nebraska biofuels plant are confirmed dead, officials said Wednesday, as firefighters were still battling smoke and flames smoldering in the wreckage nearly a day later.

“My heart hurts,” Fremont Mayor Joey Spellerberg said. “It’s a tragedy. We pray for all the families involved.”

At a news conference, Spellerberg said the two children were waiting for an employee to get off work at Horizon Biofuels so they could go to a doctor’s appointment. He wasn’t sure of their exact ages but said both were under age 12. Authorities are not releasing their names at this time.

The blast at around noon Tuesday shredded the top of the plant’s tall tower, exposing mangled concrete and rebar, while the metal walls and roof of the building below were crumpled and charred. Spellerberg said crews were evaluating whether the whole facility might collapse.

The structure’s instability has made it difficult for crews to gain access as they work to put out the blaze, said Carl Nielsen of the city’s volunteer fire department. The flames kept burning despite rain overnight.

“It’s going to be very slow,” he told reporters, saying authorities do not have a timeline for when they expect the bodies to be recovered.

The plant makes animal bedding and wood pellets for heating and smoking food, using tons of wood waste, and Fremont Fire Chief Todd Bernt has said they believe the facility stores wood and some alcohol-based materials. A 2014 fire at the building had damaged the electrical system but left the structure intact, according to reporting by the Fremont Tribune.

The mayor said he and investigators strongly suspect the blast was caused by wood dust in the elevator portion of the plant. “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.

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.

“I got up and looked outside and there was a huge plume of smoke,” she said. “We were really unsure when the explosion happened which plant it was, because there are so many in that area.”

Bernt, the fire chief, said first responders were up against “heavy smoke and a lot of flames” when they first arrived at the facility, which 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.