The flour, thought to be wheat gluten and rice protein concentrate, contained melamine to appear higher in protein, the Food and Drug Administration speculated.

The mislabeling went undetected by pet-food makers who used the ingredients, the importers who imported them and the FDA for weeks.

The FDA also said some of the contaminated product was used to make fish feed sold to U.S. fish farms. There's no evidence that any of the fish were eaten by humans, but the health risk would be low, said David Acheson, FDA's assistant commissioner for food protection.

The feed was manufactured by Skretting of Vancouver, British Columbia, and sold by a subsidiary, Bio-Oregon, which specializes in feed for young salmon, trout and steelhead. Because they take 18 months to 2½ years to grow from fry to market-size, it's unlikely any of them had been harvested.

The fish will be sampled for melamine, the chemical at the heart of the pet-food recall, Acheson said.

He speculated the wheat flour was spiked with melamine to falsely inflate its nitrogen level. That is what pet-food makers look for when checking for protein.

Wheat flour is even cheaper than wheat gluten or rice protein concentrate. They are the protein portion of the wheat or rice grain, left after the starch is washed away.

The mix-up may raise questions about the quality controls of pet-food makers. While wheat flour, wheat gluten and rice protein all come in powdered forms, wheat flour is typically whiter than wheat gluten, says Greg Aldrich, animal nutritionist at Pet Food & Ingredient Technology.

He says he'd spot the difference nine out of 10 times by looking at the product. However, he's an ingredient expert and factory workers may not be, he says. It's also unclear if the ingredients' coloring was altered.

Typically, pet-food companies test vegetable proteins for such things as mold toxins, protein and ash. If wheat flour is spiked with melamine, the protein test wouldn't reveal its true identity. The ash contents of wheat flour and wheat gluten are about the same, Aldrich says.

U.S. importer ChemNutra supplied mislabeled wheat flour to Menu Foods, which launched the recall March 16. ChemNutra also co-brokered a shipment to Canada, where it was used to make fish feed.

ChemNutra told the FDA three weeks ago about the shipment, says spokesman Steve Stern. Some of it came from the same Chinese company that supplied Menu Foods.

The FDA suspects melamine and a related chemical are creating a toxic brew that has led to pet deaths.

In addition to fish, hogs and chickens also ate salvaged pet food made with contaminated ingredients. As with the fish, the FDA says human risk is very low because small amounts of pet food went into animal feed.