Tests of an iceberg lettuce sample for the parasite that has caused an explosive diarrhoea outbreak affecting thousands of people in the US produced a false positive, officials have now said.

The Food and Drug Administration (FDA) on Saturday reported that the sample from food supplier Taylor Farms, which supplies the Taco Bell food chain, had tested positive for cyclospora.

It said on Sunday that it had re-reviewed the sample and reported the false-positive to Taylor Farms, adding it “continues working with the firm to ensure product implicated in this outbreak has been removed from the market”.

The latest update means the exact source of the parasite is still not known.

No deaths have been reported from cyclosporiasis infections, but at least 94 people have been hospitalised.

Taylor Farms announced on Friday that it was voluntarily removing all iceberg lettuce from the US market sourced from central Mexico, and issued a list of affected products.

The FDA reported the following day that the positive sample was taken from a product not part of Taylor Farms’ current recall announced on Friday.

A recall differs from a voluntary product removal in that it requires a company to notify customers and recover affected products from commerce, making it a broader and more costly corrective action.

The FDA said it collected the sample during targeted surveillance of the import as part of its investigation into the outbreak.

It said on 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 Sunday’s statement.

The infections were first detected on 13 May, according to the FDA.

Symptoms of the foodborne illness can take up to two weeks to present. They include watery diarrhoea that lasts for days, sudden weight loss, and a loss of appetite.

Thousands of people have been infected with Cyclospora linked to iceberg lettuce supplied by Taylor Farms and sold at Taco Bell locations in at least five states, according to the Centers for Disease Control and Prevention (CDC).

The CDC advises not to eat shredded iceberg lettuce served at Taco Bell locations in Indiana, Kentucky, Michigan, Ohio, and West Virginia.

The fast food chain said it had completed its removal of affected Taylor Farms lettuce from its restaurants.

In a statement, Taylor Farms said it has “stopped receiving product from the implicated lot, suspended distribution of the iceberg lettuce from Central Mexico, notified [its] customers and [is] continuing to work with the FDA, CDC, and state authorities”.

The lettuce was recalled , external in 27 states. It also includes Marketside-branded iceberg lettuce sold at Walmart locations in 15 states, according to the FDA.