On May 12, 2004, the Oregon State Public Health Laboratory identified a cluster of five patients infected with Salmonella Enteritidis (SE). The isolates of these patients stool cultures were found to be genetically indistinguishable through the use of pulsed-field gel electrophoresis (PFGE). The five patients were from four Oregon counties and had onsets of illness ranging from February to April, 2004. Further investigation would lead to documentation of at least 29 patients in 12 states and Canada with matching SE isolates, since at least as far back as September, 2003. After a thorough investigation by local, state, and federal officials, the illnesses were definitively linked to raw almonds distributed by Paramount Farms. The investigation led to the recall of roughly 18 million pounds of Paramount Farms raw almonds.

After the discovery of the matching PFGE pattern, the five Oregon patients were interviewed. All reported consuming Kirkland Signature brand raw almonds, purchased at Costco stores, and produced and distributed by Paramount Farms. Using a case-control study method it was determined that even if it was assumed that 20% of all Oregon residents ate Kirkland Signature brand almonds weekly, the chance of the outbreak being unrelated to the almonds was less than 0.1%. Interviews with the subsequently identified persons with matching SE isolates further confirmed the link to Paramount Farms. In addition, an environmental investigation revealed Salmonella to be present at Paramount from three samples from two-huller-shellers that supplied Paramount during the period of interest.
As a result, the Health Hazard Evaluation Board at the Center for Food Safety and Applied Nutirition, a part of the Food and Drug Administration (FDA), concluded that Paramount Farms raw almonds: pose an acute, life-threatening hazard to health, particularly in children and elderly persons. Additionally, any products manufactured using these almonds, where the new product did not receive an adequate kill step to eliminate Salmonella from the finished product, likewise poses an acute, life-threatening hazard to health, particularly in children and elderly persons.