A nationwide listeria outbreak that has killed 25 people who ate tainted cantaloupe was probably caused by unsanitary conditions in the packing shed of the Colorado farm where the melons were grown, federal officials said Wednesday.
Government investigators said that workers had tramped through pools of water where listeria was likely to grow, tracking the deadly bacteria around the shed, which was operated by Jensen Farms, in Granada, Colo. The pathogen was found on a conveyor belt for carrying cantaloupes, a melon drying area and a floor drain, among other places.
“You’re rolling around cantaloupe on uncleanable equipment and you’re getting it wet and you’re not cooling it — it provides the perfect environment for listeria growth and spread,” said James Gorny, a senior food safety adviser at the Food and Drug Administration.
The outbreak, which began in late July, is the deadliest caused by foodborne disease since 1985. A total of 123 people in 26 states have fallen ill, including those who died, according to the Centers for Disease Control and Prevention.
Nicholas J. Parolisi Jr., a lawyer for Jensen Farms, said he could not comment on the F.D.A.’s accusations.
The farm had passed a food safety audit by an outside contractor just days before the outbreak began. Eric Jensen, a member of the family that runs the farm, said in an e-mail that the auditor had given the packing plant a score of 96 points out of 100.
F.D.A. officials did not criticize the auditor directly. But Michael R. Taylor, deputy commissioner for foods, said the agency intended to establish standards for how auditors should be trained and how audits should be conducted.
The food industry increasingly has come to rely on what it calls third-party audits of farms or processing plants to ensure the safety of food. But the auditors are hired by the companies being inspected, and their procedures are largely unregulated. In several recent food safety lapses, the facilities involved had passed third-party audits.
It was not clear how listeria initially got into the packing shed, which officials described as an open-air structure having a concrete floor, a roof and no walls.
Listeria is frequently found in soil or manure, but tests of the soil on the farm did not turn up the bacteria. Officials said that a dump truck used to take culled melons to a cattle farm was parked near the processing shed and could have brought bacteria to the facility.
Jensen Farms, run by Mr. Jensen and his brother Ryan, had recently acquired a set of used machinery to upgrade the way it washed and dried its cantaloupes. The equipment had been used to clean potatoes and was not intended for use with cantaloupes, officials said. They said the equipment was corroded in places and built in a way that made it difficult to clean and sanitize.
An area used to dry the melons included a cloth cover that could easily have harbored the bacteria, according to a person who discussed the operation with the Jensens.
Officials also said that the cantaloupes had not been adequately cooled before they were placed in refrigerated storage, which could have caused condensation to form on the fruit, creating hospitable conditions for listeria. The bacteria grow well in wet or damp conditions and can also thrive in cold.
Dr. Gorny said that some of the conditions he described, including pools of water on the floor, had been noted during a visit in mid-September after the plant ceased operation and the equipment was dismantled. It was not clear if investigators who had visited the plant while it was still in operation saw the same unsanitary conditions.