BRENHAM, Texas (AP) — Blue Bell Creameries has told federal inspectors that it believes listeria bacteria spread at its Oklahoma plant through a drainage system.

The Texas-based ice cream company revealed the findings in documents sent to the Food and Drug Administration.

KXAS-TV in Dallas-Fort Worth reported Friday that Blue Bell says it believes the bacteria found at its plant in Broken Arrow, Oklahoma, spread through the drainage system, released into the air and then settled on nearby equipment.

The company said it couldn’t identify a single source of listeria that contaminated equipment at its plant in Brenham, Texas, but that the listeria likely entered from various potential sources.

The contamination last year was linked to three deaths in Kansas and several illnesses.

The company has since reintroduced products back into its markets.