(CNN) -- Firefighters are searching an accident site at a Nebraska industrial plant for bodies, the interim Omaha fire chief said Monday.
Bernard Kanger said there were 17 survivors and some deaths at the plant after a fire and partial building collapse.
"There are fatalities, we're not going to release the number of those fatalities at this time simply because we have not had the opportunity to clear the entire structure," Kranger said.
There were 38 people in the building at the time of the incident, Kanger said, but it was unclear how many of the missing had died because emergency workers were still going through the building looking for bodies.
"I just heard a crack pop and big ball of fire, and I just took off running when I heard the first crack," worker Jamar White told CNN affiliate KETV. "That's all I could do was get out of the way and make sure I was OK."
Of the 17 survivors, four were sent to hospitals with critical wounds while six other people had non-life-threatening injuries but were sent for treatment. Seven people at the International Nutrition plant refused treatment, leaving 21 people either dead or unaccounted for, Kranger said.
Another employee told KETV that the ceiling collapsed.
"I heard the explosion and stuff started falling, so I ducked for cover," Nate Lawis said. "It was pitch black in there. All I could see was fire. I had to feel my way out of the place. I couldn't see anything."
At least one firefighter also was injured when he cut his hand.
"Because of the significant collapse and potential risk to our individuals, we had to pull everybody out of the structure and we have to call in specialized teams that can secure and shore up the building in order to allow us to completely clear the structure and declare that we've found all of the individuals that may have been in the building at the time of this incident," Kranger said.
An urban search-and-rescue task force was activated by Nebraska Emergency Management.
The incident happened at a company that produces feed and other products for livestock and poultry about 10 a.m. CT.
Rescuers first had to to shore up the building to make it safe to begin their search, Kanger said.