All of the missing workers are now accounted for.
The blast occurred on Monday at a fuel depot connected to a refinery operated by Italian energy firm Eni.
Nine people were injured, three of them seriously.
Eni said it is still unclear what caused the explosion.
The public prosecutor’s office is now also investigating.
The blast was probably caused by a spill, according to an initial reconstruction.
Some liquid was probably spilled while tanker trucks were being refilled, investigators said.
A thick black cloud of smoke hung over the site in Calenzano, to the north-west of Florence, for hours after the accident.
The fatalities are workers aged between 45 and 62.
AAP/ANSA
