The investigation was launched after the California Department of Food and Agriculture (CDFA) said laboratory testing had detected the chemical, melamine, in urine from hogs at a farm in Ceres.
Due to the pet food scare, the findings could potentially further raise consumer fears over the safety of the food chain, even as Congress is examining whether the FDA failed to prevent recent contamination outbreaks linked to spinach and lettuce.
The 1,500 hogs at the American Hog Farm have been quarantined while the CDFA conducts additional testing to determine if melamine is present in meat.
The American Hog Farm operates a sizable part of its business through a "custom slaughterhouse," which processes animals on-site and sells them to individuals for personal use and not for resale.
The hogs were fed leftovers from the production of pet food containing rice protein concentrate. The FDA believes the melamine was present in the concentrate.
The FDA is also testing imports of cornmeal, soybean meal, corn gluten, rice protein concentrate and rice bran for the chemical's presence.
The original scare began when the chemical was found in pet food containing contaminated wheat gluten or rice protein concentrate shipped from China.
Pet food manufacturers have voluntarily recalled more than 100 brands of dog and cat food across the country since March 16, prompted by reported cases of cats and dogs that developed kidney failure after eating the affected products.
Melamine is banned in human and animal food in the US. It is also banned for use as a fertilizer, as it is in some parts of the world. Melamine can sometimes be added to the grain products to make them appear higher in protein than they are.
Due to concerns in the US, China this week banned the chemical's presence in ingredients meant for export.