The late afternoon explosions immediately killed two workmen and the 71-year-old wife of the owner of the family-run Costa company in Barcellona Pozzo di Gotto.
The owner’s son had attempted to save his mother and was one of four people taken to Palermo’s hospital with serious injuries, police said.
One of those injured later died in hospital.
Another worker who had been reported missing was later discovered dead.
One employee who was inside the depot at the time of the explosion was pulled out alive.
Firefighters were continuing to search for further victims, while the local prosecutor’s office opened an inquiry into the incident.
“According to the initial evidence, it seems that there were two explosions,” police commander Giancarmine Carusone said.
He added that it was believed sparks from welding equipment had caused the explosions.
The factory was preparing for New Year celebrations which include traditional firework displays across the south of Italy, designed to chase away evil spirits, a ritual dating back to the 1500s.
