North Carolina ? 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 sprawling 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.”