The equipment purchased in July, the same month the outbreak started was previously used to wash and dry potatoes, the agency said, and the listeria "could have been introduced as a result of past use of the equipment," according to the report.
FDA officials said that they are not concerned about similar listeria contamination in the potatoes that were previously processed on the equipment because those vegetables are rarely eaten raw. Cooking can kill the bacteria.
The agency said the contamination likely happened in the packing house, but the way the cantaloupes were cooled after being picked may have also 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.
FDA said that samples of cantaloupes in Jensen Farms' fields were negative for listeria, but bacteria coming off the field may have initially introduced the pathogen into the open-air packing house, where it then spread. Listeria contamination often comes from animal feces or decaying vegetation.
Another possible source of contamination was a truck that frequently hauled cantaloupe to a cattle operation and was parked near the packing house. Contamination could have come from the cattle operation and then tracked into the house by people or equipment, the report said.
The Centers for Disease Control and Prevention says 123 people have been sickened in the outbreak, including the 25 who died. 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 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.