A US federal agency hasn’t still identified the source of an E. coli outbreak that made at least 19 people sick in seven states. They might have got infected after consuming rotisserie chicken salad sold at Costco Wholesale Corp.

On Tuesday, the agency said that a US Food and Drug Administration (FDA) laboratory analysis hasn’t confirmed E. coli presence in a sample of celery and onion mix taken from a Costco store in Montana.

This is the latest E. coli occurrence incident wherein the source of the outbreak isn’t known. Health officials have so far failed in identifying the cause of an E. coli outbreak at Chipotle Mexican Grill Inc owned restaurants in 9 states.

The celery and onion mix was prepared by Taylor Farms for use in the salad, in the Costco outbreak.

Taylor Farms Pacific Inc recalled 71 products sold at retailers like Safeway and Wal-Mart Stores Inc voluntarily in November citing ‘an abundance of caution’ as the mix tested positive for E. coli in a sample collected by the Montana Department of Health.

The state carried out a rapid preliminary test, sending a partial sample of the mix of celery and onion to the FDA. The FDA then carried out a more exact test that involved growing a colony of the bacteria.

Among 16 people, 14 people bought or consumed rotisserie chicken salad from Costco in a week before the illness began. The shares of Costco faced a slight change at $169 in after-hours trading.