The outbreak is blamed for 25 deaths, and 123 illnesses. It is the deadliest known outbreak of foodborne illness in the U.S. since an outbreak of listeria in Mexican-style cheese in 1985.
The listeria was earlier traced back to a cantaloupe farm in Colorado.
They also said there was listeria in pools of water on the floor of the Jensen Farm work area.
The FDA said Jensen Farms had recently purchased the used packing equipment blamed for the contamination. The equipment - purchased in July, the same month the outbreak started - was previously used to wash and dry potatoes.
Agency officials said they are not concerned about listeria contamination in the potatoes previously processed using the equipment because potatoes are rarely eaten raw. Cooking can kill the bacteria.
According to the FDA findings, the contamination likely happened in the packing house and the cooling process also may have contributed to listeria growth.
The farm did not use a process called "pre-cooling" that is designed to remove some condensation, thus creating moist conditions on the cantaloupe rind that are ideal for listeria bacteria growth. Listeria grows in cool environments, unlike most pathogens.
The FDA also said another possible source of contamination was a truck that hauled cantaloupe to a cattle operation and parked near the packing house. Contamination could have come from the cattle operation and then tracked into the house by people or equipment.
The tainted fruit, which Jensen Farms recalled in mid-September, should be off store shelves by now. But the number of illnesses may continue to grow - symptoms of listeria can take up to two months to appear.
The Associated Press contributed to this report.