The United States Department of Agriculture (USDA) is responsible for making sure that animals that are slaughtered are done so in a humane way, in accordance to the animal welfare laws that cover slaughter plants. However, it seems that animal welfare is not being taken seriously by those who work in these plants, which leads to violations of these laws.
http://forcechange.com/68701/enforce-animal-welfare-laws-at-slaughter-plants/