The manufacturers of Blue Bell Ice Cream are under investigation by the US Department of Justice (DOJ) over the Listeria outbreak that shut down production of the confectionery treat last April, according to CBS News.

During an investigation by the Food and Drug Administration (FDA) earlier this year, Listeria was found in all three of Blue Bell’s production facilities in Alabama, Oklahoma, and Texas, prompting a shut down of the production plants and a recall of all Blue Bell ice cream.

Plant records indicated the company knew of the contamination as early as 2013 in at least one of the facilities.

The FDA investigation also found some other unsanitary conditions present, including condensation dripping directly onto the ice cream during processing at the Texas plant and other incidents of unsanitary equipment.

The violations found at the facility in Oklahoma included dirty equipment, inadequate food storage, food being held at improper temperatures and employees not washing hands adequately.

The Alabama plant was cited for employees wearing soiled clothing.

Facility workers had previously described unsanitary conditions to CBS News, such as rain water from the roof dripping into the ice cream and other unclean conditions at the plants, and the whistle-blowers said their complaints to management went unheeded.

The DOJ investigation wants to know when Blue Bell management knew about the conditions and the Listeria contamination and what they were doing to clean it up.

The recall last April resulted in all Blue Bell ice cream being pulled from the shelves, and the ice cream was associated with sickening seven people and three deaths in Arizona, Texas, Oklahoma and Kansas.

During the initial investigation by the company, a spokesman said they had traced the Listeria contamination down to two factories, the one in Oklahoma and the one in Texas, but added they were not sure how the contamination entered the facilities.

All of Blue Bell’s facilities are now up and running and ice cream is being re-stocked on the shelves.