Some information about ingredient standards for pet foods.
Information about “human-grade”.
And here is one giving a summary of ingredients.
So, in essence, if the animal ingredients for a pet food all come from a USDA approved slaughterhouse, it could be considered human edible. Except, they may include food that has been rejected for human consumption (aesthetics or just organs not sold in the US like lungs).
Using animal ingredients from animals that were not slaughtered usually need that those products be further processed. These are not considered edible for humans.
Because of safety reasons, pet food is expected to be free from food-borne bacteria like Salmonella, otherwise there are recalls (just like with human food).