The ice cream company said the new discoveries show its testing procedures are working, and it is now extensively cleaning and sanitizing the suspect areas.
Blue Bell emphasized that it has found no confirmed cases of listeria
"The entire purpose of our enhanced environmental testing is to identify locations where bacteria could be found in our facility in order to properly clean and sanitize the surface and prevent contamination," Blue Bell said in a statement Friday.
Texas-based Blue Bell recalled all of its frozen products sold in 23 states
in April because they had the potential to be contaminated with listeria. It has been gradually returning its products to store shelves, with the last phase of market reentry set for later this month
"We have tested and will continue to test every batch of ice cream produced," the company said. "No products produced have tested positive (for listeria). No products are shipped to stores until tests confirm they are safe. We will continue to work closely with our regulatory agencies, as we have throughout this process."
Blue Bell products linked to fatal outbreak
An investigation by the Centers for Disease Control and Prevention
indicated that various Blue Bell products were the source of an outbreak of listeriosis that sickened at least 10 people and led to three deaths.
The CDC said some of the cases were as far back as 2010. It connected the patients through comparisons to a database of bacteria DNA.
Listeriosis is a potentially fatal infection caused by the germ listeria, which is common in the environment. Unlike other germs, however, listeria can grow even in the cold temperature of a refrigerator -- which may explain why the cases spanned years.
"The fact that it was the same strain over the last five years suggests it could have lurked somewhere in the factory the whole time," Dr. Robert Tauxe, deputy director of the CDC Division of Foodborne, Waterborne and Environmental Diseases, told CNN last April.
Blue Bell is under criminal investigation by the U.S. Department of Justice
, which is examining whether company executives committed wrongdoing in their handling of the outbreak, a U.S. official confirmed to CNN this month.
Approximately 1,600 cases of listeriosis are reported in the United States every year, with 260 resulting in death, according to the CDC.