The US Department of Justice (DOJ) is investigating Texas-based Blue Bell Creameries for its handling of last year’s Listeria outbreak that was traced to a contamination at company plants, forcing a nationwide recall of its ice cream products. The outbreak was linked to 10 cases of listeriosis, including three deaths that are believed to be associated with the contamination, according to CDC.

The DOJ’s investigation into Blue Bell will seek to determine exactly what company executives knew about the hazards at their plants and when they knew them, sources told CBS news.

A Listeria contamination was found at all three Blue Bell plants—Texas, Oklahoma and Alabama—during last year’s FDA investigation, though FDA documents detailed 17 presumptive positive tests for Listeria at the Oklahoma plant dating back to 2013. However, according to CNN, a CDC investigation showed the listeria outbreak dated back to 2010.

Attorney James Neale, who is not involved in the Blue Bell case, told the Wall Street Journal “It's no longer unusual for the Justice Department to take an interest in an outbreak of this magnitude.”

Blue Bell recalled all of its ice cream in April last year, shutting down production at all plants. After the investigation, FDA found the source of contamination at the Oklahoma plant likely came from condensate dripping onto equipment. Production slowly resumed beginning in August 2015 and the company is currently in the middle of a five-step market re-entry plan that is restoring products back to store shelves. Approximately 1,000 furloughed workers, company-wide, were brought back to their jobs by the end of 2015.