GARNER, N.C. -- Two workers died when an explosion tore through a Slim Jim meat products plant in North Carolina on Tuesday, punching holes in the building's roof and blowing employees off their feet.

The explosion critically burned four people and one worker was still unaccounted for Tuesday night. More than 40 others were taken to hospitals, including three firefighters who needed medical attention after inhaling ammonia gases that left a distinct scent around the ConAgra Foods Inc. plant just south of Raleigh.

The blast collapsed an exterior wall, smashing cars parked next to the 500,000-square-foot building.

Authorities could not say where in the plant the blast happened or what caused it, but some of the more 300 workers on duty said it chaos and panic followed.

"I was getting ready to pick up a piece of meat off the line and I felt it -- the percussion. And you could feel it in my chest and my ears popped," said worker Chris Woods. "One of the guys I was working with got blown back -- his hat flew backwards."

Crews searched for three missing workers through the day. Searchers were in part relying on cameras and listening equipment. They were also concerned because the building was unstable, and the bodies were not immediately removed.

Frank McLaurin, a battalion chief with North Carolina Task Force 8 Urban Search-and-Rescue, said teams crawling through spaces no higher than 30 inches would work until the last person was found.

Janelle Lynch, who has worked at the plant for eight years, said she saw flames and ran. She planned to leave through the cutting department, but the roof started to collapse, so she went in the other direction and escaped through a warehouse.

"I saw a fire and things just started exploding," she said.

ConAgra spokesman Dave Jackson said someone called the plant over the weekend and threatened to start a fire. He said company officials don't believe the threat was connected to the explosion, but Garner Police Sgt. Joe Binns would not say whether police think there is a link.

"I don't want to go in that direction right now," Binns said. "We're focused on the rescue, not the investigation."

Four people were in critical condition at UNC Hospitals with burns covering between 40 percent and 60 percent of their bodies, said Dr. Charles Cairns, professor and chairman of the department of emergency medicine at the University of North Carolina.

"Anything that covers more than 50 percent of the body surface area is a very major burn and can be complex to take care of and can result in major complications, including death," Cairns said. "So these people are very severely burned."

Debbie Laughery, a spokeswoman for WakeMed, said its hospitals in Raleigh and Cary were treating 17 patients, including two in serious condition, for broken bones, bruises and burns consistent with blast injuries. She said some complained of burning in their throats.

The Environmental Protection Agency was on scene to monitor the air but officials said there was no threat to people who live near the plant, which is located between Interstate 40 and a subdivision but mostly surrounded by large buffers of trees.

Crews kept people away from the site as smoke puffed through holes in the damaged roof early in the day.

About 900 people cover four shifts at the plant, one of ConAgra's largest, Jackson said. The ammonia is used to refrigerate meat before it's turned into Slim Jims.

ConAgra spokeswoman Stephanie Childs said the company will continue to pay its Garner employees indefinitely.

The company, which has 25,000 employees worldwide, makes brands like Chef Boyardee, Hunt's tomato sauce, ACT II popcorn and Hebrew National hot dogs.

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. The plant had violations in previous years, including a fine in 2007 for problems with eye and face protection equipment.