A listeria outbreak linked to frozen nutritional milkshakes is to blame for 12 deaths and dozens of hospitalizations, the U.S. Food and Drug Administration confirmed Monday following an investigation that spanned years.

A range of frozen milkshakes have been recalled after they were linked to 11 deaths and dozens of serious illnesses, many of them in long-term care homes and hospitals, the FDA confirmed on Friday.

The supplement shakes were sold to hospitals and long-term care facilities, including nursing homes. There is a nationwide recall for the ReadyCare and Sysco Imperial Frozen Supplemental Shakes. Twelve deaths associated with the listeria outbreak have been linked to the shakes.