WASHINGTON – The source or sources of an outbreak of food-borne illnesses caused by Cyclospora continues to challenge federal and state investigators. As of Sept. 20, the Centers for Disease Control and Prevention said there have been 643 reports of Cyclospora infection from 25 states. Yet only those illnesses traced back to the states of Iowa and Nebraska have been linked to a source.

Public health officials in Iowa and Nebraska performed investigations within their states and concluded that restaurant-associated cases of Cyclospora infection in the states were linked to a salad mix produced by Taylor Farms de Mexico, a business unit of Taylor Farms, Salinas, Calif. The C.D.C. said it is collaborating with the Texas Department of Health and Human Services to investigate cases of cyclosporiasis reported in the state.

An analysis of the results from the investigation into a cluster of illnesses that have been sourced to a Texas restaurant did not show a connection to Taylor Farms. The C.D.C. said the investigation is ongoing and that based on the available evidence not all of the cases of cyclosporiasis may be related to each other.

The Food and Drug Administration has dedicated a 21-person team at the headquarters level to finding the source or sources of the outbreak. In addition, the agency said it has assigned F.D.A. specialists in field offices throughout the country to the investigation.

The F.D.A. said it has prioritized the ingredients contained in the salad mix identified as the source of the Iowa and Nebraska cases for its trace-back investigation, but is following other leads. In the trace-back process, the investigators identify clusters of people made ill in separate geographic areas and works to trace the path of food eaten by those made ill to a common source. The agency said the process is labor intensive, requiring the collection, review and analysis of hundreds and at times thousands of invoices and shipping documents.