Jeffrey Hammerstein, district chief with Wake County Emergency Medical Services, said five injured people were tagged as priority patients with serious conditions.
Hammerstein said officials were searching for three people missing from the site in Garner, a Raleigh suburb, but it wasn't clear whether they were inside during the explosion.
Garner Mayor Ronnie Williams said injuries ranged from burns to smoke inhalation. Emergency crews were keeping people away because of concerns about the smell of ammonia.
"There is a toxic cloud on and about the plant," Williams said.
Officials said at a news conference that firefighters were still trying to contain a small fire and an ammonia leak.
The site produces Slim Jim products. Williams said several people reported hearing an explosion before the building partially collapsed.
"We're still working evacuating anybody else that was still in the building and treating people that were injured," said Lt. Chris Hagwood, a spokesman for the Garner Police Department.
About 900 people work at the 50,000-square foot plant, which is considered one of ConAgra's largest, said Dave Jackson, a spokesman for the Omaha, Neb.-based company. He said he did not yet know how many people were in the plant at the time of the collapse and the company was not sure about the cause yet.
"Obviously our first priority is the safety of our employees and the community and making sure our employees are accounted for and working with them to get them whatever they might need," he said.
ConAgra is sending a team of experts to the facility and is helping local authorities.
ConAgra Foods Inc. makes brands like Chef Boyardee, Hunt's tomato sauce, ACT II popcorn and Hebrew National hot dogs. It has 25,000 employees worldwide.
The plant last was inspected by the North Carolina Department of Labor for workplace safety last July and no violations were found, said Labor Department spokeswoman Dolores Quesenberry.