FDA officials say further laboratory analysis of the celery and onion salad mix samples used in Costco's rotisserie chicken salad could not confirm the presence of E. coli.

Health officials in Montana previously identified the salad mix—produced by Taylor Farms Pacific, Inc. of California—as the source of an bacteria contamination in a chicken salad product sold at Costco stores linked to a multistate E. coli outbreak that sickened 19 people in seven states. As a result of the lab tests in Montana, Taylor Farms voluntarily recalled 71 products last month citing “an abundance of caution.”

According to FDA, the recalled products were primarily used to make sandwiches, salads and wraps and were sold at retailers including Target, Walmart, Safeway, Raley’s and others in addition to Costco.

But federal officials now say they are unable to replicate these results, though this does not mean the celery and onion mix is ruled out as the source. Part of the challenge with identifying the exact ingredient comes from the polymerase chain reaction test Montana officials used in the preliminary tests. These tests are commonly used to rapidly screen for the presence of bacteria such as E. coli, though positive results may not always be confirmed because “other bacteria that may be present in the sample can interfere with the growth and isolation of the bacteria in question, or the bacteria in question may be in very low numbers and hard to isolate or perhaps have died off by the time confirmation testing was done,” according to FDA.

Montana health officials stand by their results, telling the Associated Press they definitely found E. coli DNA in pieces of the vegetable mix sample they took from Costco.

As of Dec. 8, 19 people with this outbreak strain of E. coli were reported to be infected in seven states, including Washington, California, Montana, Utah, Colorado, Missouri and Virginia. The CDC investigation found 14 of the infected people purchased or ate the chicken salad product in the week before their illness started.

FDA and CDC say the investigation is ongoing and they are working to identify the specific ingredient in the chicken salad linked to the illnesses.