Government investigators found positive samples of listeria bacteria on equipment in the Jensen Farms packing facility and on fruit that had been held there.
In a six-page assessment of the conditions at the farm based on investigators' visits in late September, the FDA said Jensen Farms had recently purchased used equipment that was corroded, dirty and hard to clean. The packing facility floors were also constructed so they were hard to clean, so pools of water potentially harboring the bacteria formed close to the packing equipment.
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. Barbara Mahon of the CDC said that the illnesses peaked from mid-August through September, but that the government would continue to monitor the situation for at least another two weeks.
The CDC on Tuesday confirmed a sixth death in Colorado and a second in New York. Indiana, Kansas, Louisiana, Maryland, Missouri, Nebraska, New Mexico, Oklahoma, Texas and Wyoming have also reported deaths.
FDA officials said Wednesday that the agency has never visited the farm to do an inspection. But that would likely change under a new food safety law signed by President Barack Obama earlier this year that boosts the number of inspections the FDA conducts annually. Currently, the agency may only visit a food facility every five or 10 years, at the most.
FDA officials said they have visited many food facilities over the years and the conditions at Jensen Farms were unique.
"There is no reason to believe these practices are indicative of practices throughout the industry," said Sherri McGarry, a senior officer in FDA's office of foods.
McGarry said the agency has sent the company a warning letter and is still considering what enforcement actions it will take. Officials said the farm had cooperated in all aspects of the recall and investigation.
Messages left for the farm's owners were not immediately returned.
Associated Press Writer P. Solomon Banda contributed to this report from Denver.