A combination of safety lapses inside the packing plant at Jensen Farms in Colorado likely led to the contamination of cantaloupes blamed for the nationwide outbreak of at least 123 Listeria illnesses, including 25 deaths, the Food and Drug Administration said Wednesday.

In a media briefing and a formal assessment of conditions at the farm, FDA officials said samples taken from the Jensen Farms fields and irrigation water, as well as cantaloupes picked directly from the fields, were negative for the deadly outbreak strains of Listeria.

But samples collected inside the packing plant, as well as cantaloupes in cold storage, were positive for the bacteria.

While not spelling out precisely how Listeria, which can be in soil and animal feces, came into the plant and harbored there, FDA officials pointed to several factors they say likely caused Listeria to grow and spread inside the packing plant to lethal levels:

— A truck used to haul culled cantaloupes shuttled to and from a cattle operation. Employees may have tracked Listeria into the plant from the truck, which was parked adjacent to the packing facility.

— Water was pooled on the floor next to equipment and employee walkways inside the packing plant. Listeria harbors in standing water and wet places, and perhaps spread to conveyors and other packing equipment. Samples collected from the pooled water tested positive for an outbreak strain of Listeria.

— The packing equipment could not be easily cleaned and sanitized. Inspectors saw “visible product buildup” on food contact surfaces.

— Recently purchased used washing and drying equipment was corroded and hard to clean, and previously had handled other raw produce. 

— Natural moisture or increased water from washing the cantaloupes could have encouraged the growth of Listeria on the cantaloupe rind. Moreover, melons warm from the field were not pre-cooled before going into cold storage, so the condensation on the melons likely created conditions ideal for Listeria growth.

The FDA officials said the findings at Jensen Farms highlight the importance of following the government’s existing guidelines on minimizing microbial hazards in fresh fruits and vegetables. They also said the report shows the need for growers to periodically evaluate their processes and equipment to make sure they are not contributing to the contamination of fresh produce.