Yum! Brands (YUM) Taco Bell outlets in the US are still under investigation due to concerns that lettuce sourced from Central Mexico recently triggered the largest US food-borne illness outbreak in recent years, according to multiple news outlets Tuesday.

A recent FDA test of sample lettuce from the privately held Taylor Farms, which farms in Mexico and is a Taco Bell vendor, proved to have been a false positive for the cyclospora parasite. The pathogen, when ingested, can cause diarrhea, the reports said.

However, an FDA scientist said that investigations into the Mexican lettuce are justified, due to epidemiological and traceback evidence that stands on its own, the reports said.