BRENHAM, Texas (AP) — Blue Bell Creameries told federal inspectors that it believes that listeria bacteria spread at its Oklahoma plant through a drainage system, but the company said it couldn’t identify a single source of listeria that contaminated equipment at its flagship facility in the Central Texas town of Brenham.Related:Blue Bell ice cream returns to Knoxville

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.

In its filing to the FDA, Blue Bell said cleaned equipment that came into contact with products made at its Broken Arrow, Oklahoma, plant was being stored in a small room with a floor drain. The company said an investigation revealed that particles at the plant may have carried listeria and washed into the building’s drainage system, gone through the room’s drain and settled on the clean equipment.

“We believe that this mechanism — particles emitted from a drain — was the most likely source of listeria,” Blue Bell wrote in the February filing.

The company said it no longer uses the room for equipment storage, and that the drain was removed and the floor replaced.

Blue Bell also told the FDA that it believes listeria likely entered its Brenham plant from “various potential sources” and settled on some pieces of equipment, but that investigators “could not identify a single common source of listeria in the facility.”

The company said it focused on cleaning affected pieces of equipment or removing it altogether, cleaning and sanitizing the plant and enhancing sanitation procedures and testing programs.

“We believe that these enhanced programs are enabling us to effectively control for listeria in our Brenham facility,” Blue Bell said.

Blue Bell also implemented new procedures, facility enhancements and employee training at its Alabama plant.

Last April, Blue Bell recalled its products after its ice cream was linked to 10 listeria cases in four states, including three deaths in Kansas. Listeria can cause serious illness, especially in older adults, pregnant women and people with weakened immune systems.

The company has since reintroduced its products back into its markets.