The Department of Justice has started an investigation into Blue Bell after their ice cream was linked to a deadly Listeria outbreak earlier this year that killed three people.

An FDA investigation found Listeria in all three of Blue Bell's production plants located in Alabama, Oklahoma and Texas. Records indicated that the company knew one plant was contaminated at least as early as 2013.

The FDA investigation uncovered other troubling problems, including condensation dripping directly into ice cream and unsanitary equipment. Last April, Blue Bell shut down all three production facilities, and all ice cream was recalled.

Sources tell CBS News that the Department of Justice is trying to determine what Blue Bell management knew about potentially deadly hazards in their plants, and when they knew it.

The most extensive violations were found in Oklahoma, where the FDA released 16 separate positive tests for listeria on equipment and in ice cream from March 2013 through January 2015. CBSNews.com

Last October, Gerald Bland who worked at the Blue Bell factory in Brenham, Texas, described to CBS News, unsanitary conditions on the factory floor.

"On the wall by the 3-gallon machine, if it had rained real hard and water sat on the roof, it would just trickle down," Bland said.

Rain water from the roof would leak into the factory.

Another worker, Terry Schultz, told us that his complaints to management about unclean conditions went nowhere.

"The response I got at one point [from management] was, 'is that all you're going to do is come here and bitch every afternoon?'"

The message Schultz took management's response was, "Production is probably more important than cleanliness."

All three of Blue Bell's plants are now back up and running, and by the end next month, its ice cream will be back on the shelves in 15 states.