Police said an unidentified gunman opened fire killing five people before committing suicide.
Eight other employees at the plant outside Meridian town suffered gunshot wounds and were transported to local hospitals, Lauderdale County Sheriff Billy Sollie said in an interview to CNN.
Police said the gunman was an employee at the plant and had used a shotgun and an automatic weapon in the rampage. Investigators do not know the motive for the shooting.
Earlier reports indicated the gunman was wearing camouflage clothing when he entered the plant at about 1100 EDT (1500 GMT) and began firing randomly at co-workers.
"Nothing like this has ever happened here"
The incident was the latest in a series of workplace shootings in the United States and the deadliest since seven people were gunned down at an Internet consulting firm in Massachusetts on 26 December, 2000.
Residents in Meridian, a town about 135 km east of the state capital Jackson, were stunned by the massacre.
"Nothing like this has ever happened here," Meridian Mayor John Smith told CNN. "If we have three to four violent deaths in a year, that's a surprise to us. Something like this is unimaginable to the people of Meridian."