From the Pages of Washington State University VetMed Department:

Salmonella Typhimurium is more cosmopolitan, commonly infecting a wide range of animal species worldwide. Of the 19,483 salmonella isolates obtained from agricultural animals in the United States that were submitted to the USDA for typing during a recent year, 23% were Salmonella Typhimurium, the most common serovar. During that year, Salmonella Typhimurium was the most frequently identified serovar submitted to the USDA from clinical cases in cattle, horses and pigs and was the fourth and sixth most common serovar isolated from turkeys and chickens respectively. Of the 82 different serovars submitted from clinical cases in cattle that year, it comprised fully 60% of the 2,050 submissions.

A similar serovar pattern exists in humans. Of the 9,787 human foodborne infections detected in the seven states covered by the CDC FoodNet surveillance system in 1998, 29% were due to Salmonella serovars. Salmonella was the second most frequent species, the first being Campylobacter, comprising 41% of the isolates. Of these Salmonella isolates that were serotyped 30% were Salmonella Typhimurium, the most common serovar found.

Of note is that the largest known foodborne Salmonella outbreak in the U.S. was due to Salmonella Typhimurium and that the food involved was pasteurized milk contaminated by an in-plant failure post-pasteurization.

It it doubtful that many large animals are wandering though the manufacturing facility, or that they are mixing in raw milk.  Someting more likely was found in The New England Journal of Medicine where it concluded that rodents probably are a source of human Salmonella Typhimurium infection.  Birds have also been found to be a vector.  My bet is that the source of the Salmonella will be either rat or bird poop.