Three people remain unaccounted for following a large explosion at a Nebraska plant on Tuesday, authorities said.

The incident occurred midday Tuesday at Horizon Biofuels, a manufacturer of fuel pellets and animal bedding located in Fremont, located approximately 40 miles northwest of Omaha.

“When we arrived on scene, we had heavy smoke and a lot of flames coming out,” Fremont Fire Chief Todd Bernt said during a press briefing Tuesday afternoon. “We had a lot of structural collapse.”

Three people were inside the plant at the time of the explosion, Fremont Mayor Joey Spellerberg said during the briefing, adding, “But that’s all we could say at this point.”

Due to the structural collapse, fire crews have been unable to access the building to conduct a search, Bernt said.

“Early on, we tried to get access, but just due to the collapse with the mangled steel, we were unable to get inside,” Bernt said.

Due to the structural issues, the fire department requested support from Nebraska Task Force One, which is assessing how to safely conduct a search, the chief said.

The fire remained active Tuesday afternoon, especially in the facility’s tower, according to Bernt, who said he expects fire crews to be on site for several days.

Bernt said the plant has a lot of wood and some alcohol-based materials stored inside.

The cause of the explosion remains under investigation. The Nebraska State Fire Marshal’s Office and other agencies are on scene.

Spellerberg said multiple agencies have responded to the explosion.

“It’s quite incredible to see the work, and we’re doing everything we can with this situation,” he said.

Nebraska Gov. Jim Pillen said he is closely monitoring the incident.

“Our thoughts and prayers are with all involved — and we’re ready to help any way we can,” he said in a statement.

One woman who came out to the scene to give out water to first responders said she saw a “large black puff of smoke” following the explosion.

“I was just instantly praying that nobody was hurt,” she told reporters. “That’s all we can hope for. We’re a good community. Everybody will rally together and help each other.”