U.S. investigators are visiting fish farms after learning of at least one plant that used feed tainted with melamine — a chemical used to make plastics and fertilizers, the U.S. Food and Drug Administration said Tuesday.

The FDA cautioned that the levels of contamination were likely too low to pose a significant health risk to humans who consumed the fish. U.S. officials did not say whether any of the farmed fish given the contaminated feed entered the food supply, but noted that the fish of at least one firm were too young to be sent to market.

The FDA said the Canadian-made feed included what was believed to be wheat gluten imported from China, said Dr. David Acheson, the FDA's assistant commissioner for food protection.

Acheson said while U.S. companies had used the contaminated ingredient in pet food, a Canadian company used it to produce fish meal. He also noted the U.S. agency has notified Canadian officials about the contamination.

The FDA says they will test fish samples for melamine to determine their next course of action.

Acheson also said that shipments of wheat gluten and rice protein concentrates from China were in fact lots of simple wheat flour spiked with melamine, which falsely boosted the nutrient content.

"What we discovered is these are not wheat gluten and rice protein concentrate but in fact are wheat flour contaminated by melamine," Acheson said.

Pork, chicken, egg products safe to eat: FDA

On Monday, the FDA said pork, chicken and egg products from animals that were given feed tainted with low doses of melamine did not pose a health risk to humans.

Food safety concerns grew after U.S. officials found that some small manufacturing plants had been incorporating pet food into their animal feed before tests showed the pet food was contaminated.

A massive recall involving more than 100 brands was issued on March 16 and the FDA has confirmed the deaths of 16 pets. The FDA has fielded about 17,000 consumer calls about related pet illnesses.

Canada's food inspectors have issued border lookouts for vegetable proteins coming from China to prevent melamine from contaminating the human food chain.

With files from the Associated Press