A sample of lettuce from Taylor Farms that had tested positive for cyclospora was a false positive, the Food and Drug Administration said Sunday.

“Due to the complexity in detection of Cyclospora, FDA laboratory experts re-reviewed the sample results and have concluded that the finding does not represent true amplification and should be considered a false positive,” the FDA said.

In the statement announcing the false positive, the FDA said that as of Sunday it had not found any “confirmed positive sample results for product testing for Cyclospora.”  

Taylor Farms on Friday said it was “voluntarily removing” iceberg lettuce sourced from central Mexico after the lettuce had been linked to one of the largest outbreaks of cyclosporiasis in the U.S. in recent years.   

“While the FDA traceback is indicating a specific independent farm, which represents less than 1% of the U.S.’s iceberg lettuce supply, as the potential source of the outbreak, we have removed all iceberg lettuce from the region indefinitely,” the company said Friday in a statement.

Taylor Farms said Sunday in a statement after the false positive was announced that, “Our thoughts remain with everyone who has fallen ill in this outbreak. We are committed to working with public health authorities as the ongoing outbreak investigation continues.”

On Thursday, the U.S. Centers for Disease Control and Prevention identified lettuce supplied to select Taco Bell restaurants as a source of the multistate outbreak, more than a month after cases first began to appear.

The iceberg lettuce was distributed to Taco Bell restaurants in Indiana, Kentucky, Michigan, Ohio and West Virginia, according to the CDC. Taco Bell said on Friday it had “completed removal of affected Taylor Farms lettuce from our restaurants.”

Walmart also announced over the weekend it had removed four Marketside bagged iceberg lettuce salad products from its stores in more than two dozen states, but added there was “no indication that products sold in our stores are affected by the current Cyclospora investigations.”

The outbreak, first reported in May, has spread to 34 states and sickened more than 1,600 people, according to the CDC. However, the CDC’s tally lags behind state health departments, which have reported much larger numbers of cases. For instance, the Michigan Department of Health and Human Services reported more than 5,000 confirmed cases as of Friday.

No deaths have been reported. 

Cyclosporiasis is a food- and waterborne illness caused by the cyclospora parasite, which causes long-lasting, watery and sometimes explosive diarrhea, according to public health experts. 

Outbreaks typically occur in late spring and summer and have historically been linked to bagged salads, cilantro, basil, raspberries, snow peas and green onions.