The Food and Drug Administration said on Friday its inspectors had detected cyclospora at the Mexican farm blamed for the largest foodborne illness outbreak of the parasite in US history.

FDA inspectors said Friday they found cyclospora in two places: a wastewater tank and a drainage ditch used for lettuce cultivation. They said it was unclear how the lettuce became infected.

The agency’s announcement comes less than a month after the US Centers for Disease Control and Prevention (CDC) declared an end to the multi-state outbreak, which infected nearly 13,000 people across 21 states and led to 570 hospitalizations and two deaths.

It also follows an announcement from regulators at Mexico’s own food safety authority, which conducted its own investigation and said cyclospora was not detected in the lettuce or water at the facility. That conclusion came days after the FDA was forced to retract a faulty test result that had incorrectly identified cyclospora in a sample of lettuce from Taylor Farms collected at the border.

The FDA announcement comes more than six weeks after the agency sent its investigators to the farm in Guanajuato. The visit was the agency’s first time there in more than seven years following a steep drop in international food safety inspections after the Covid pandemic.

While it’s unclear how the contamination took place, the FDA claimed the samples provide “strong epidemiological data” linking the outbreak to the Taylor Farms site.

In a separate statement, Taylor Farms attributed the contamination cited by FDA to an independent grower “who is no longer part of our supplier program”.

“Our facility remains closed in central Mexico as we work with our research team to implement enhanced safety protocols from the field to the facility,” the company said.

Cyclospora is a microscopic parasite that commonly causes watery diarrhea “with frequent and sometimes explosive bowel movements”, according to the CDC. Other symptoms included loss of appetite and weight, stomach cramps, bloating and increased digestive gas.

The illness, called cyclosporiasis, is less common than foodborne illnesses caused by other germs, including salmonella and E coli.