Garner, N.C. — The three victims in the ConAgra Foods plant explosion last month died from head and chest injuries, according to autopsy results released Thursday.

Investigators said they believe a natural gas line to an industrial water heater that was being installed in the Jones Sausage Road plant  sparked the June 9 explosion, which blew out a wall of the plant and caused part of the roof to collapse.

Barbara McLean Spears, 43, of Dunn; Rachel Mae Poston-Pulley, 67, of Clayton; and Louis Junior Watson, 33, of Clayton, died in the blast. Dozens of other workers were injured.

The medical examiner's reports show that Watson, Spears and Poston-Pulley all suffered blunt-force head trauma when debris from the plant fell on them.

Employees returned to work at the plant Sunday. The plant is the only place where Slim Jim beef jerky products are manufactured.