A cyclospora outbreak has now spread to nine US states, according to health officials, as federal investigators continue working to identify the source. The outbreak comes amid what has been described as the highest number of reported cyclosporiasis cases in US history.

The Centers for Disease Control and Prevention has linked some of the illnesses to lettuce from central Mexico. Reported cases tied to that product have been identified in Illinois, Kansas, Oklahoma and Pennsylvania, with additional cases bringing the outbreak total to nine states.

Cyclospora is a parasite that causes cyclosporiasis, an intestinal illness that can lead to prolonged digestive symptoms. The latest update adds to concern about the scale of the outbreak, especially as investigators are still tracing how the contaminated food entered the supply chain.

Officials have not yet announced a final source for all of the reported illnesses. For now, the CDC and other public health agencies are continuing their investigation as the number of affected states grows.