OMAHA, Neb. (AP) — Omaha's fire chief said Monday that people have died in an explosion and partial building collapse at an animal feed processing plant, but he would not give a specific number of deaths.

Interim Omaha Fire Chief Bernie Kanger said crews have stopped rescue efforts and will start a slower recovery effort to retrieve victims. The International Nutrition plant is unstable, so rescuers must work deliberately to ensure their safety, he said.

Thirty-eight people were inside the building when the explosion occurred Monday morning. Ten were taken to hospitals and four are in critical condition. It's unclear how many people got out without being hurt.

The cause of the blast has not yet been determined, but Kanger said there were no hazardous chemicals at the plant.

Plant worker Nate Lewis, 21, said he was on the first floor when he heard the blast. The building went dark, so he used light from his cellphone to make his way across the production floor to safety outside.

"I was a production line worker, although I don't know if I want to be that anymore," said Lewis, who's worked at International Nutrition for about four months.

There appears to be structural damage to the top of the building, which sits in an industrial area visible from Interstate 80, which bisects Nebraska's largest city. There are no residences nearby and no other buildings were evacuated after the explosion.

Diane Stout said she'd heard from her husband, a manager on the plant maintenance crew, so she knows he's OK, but she and her two daughters were still anxiously waiting to see him because he has heart problems.

The workers all know each other well, Stout said, so she was hoping to hear good news about friends there.

Sarah White said she was at home with her four children when she got a call from her husband right after the explosion.

"I could hear the panic in his voice," she said. "But he said he was OK." She said he'd been watching trucks unload from outside the building when the blast occurred.

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"That's where he works every day. That could have been him," White said, referring to the injured workers.

Her husband, Jamar White, said he was lucky.

Jamar White said he heard a loud crack and then looked up to see the back wall of the building collapsing.

"I ran at least 150 feet," White said. "I ran far enough to make sure nothing else would keep falling."

Afterward, White said, he could see inside the third floor of the building where at least two co-workers were screaming for help.