April 19, 2007  -- For the first time, investigators are saying the chemical that has sickened and killed pets in the United States may have been intentionally added to pet food ingredients by Chinese producers.

Food and Drug Administration investigators say the Chinese companies may have spiked products with the chemical melamine so that they would appear, in tests, to have more value as protein products.

Officials now suspect this possibility because a second ingredient from China, rice protein concentrate, has tested positive for melamine. So has corn gluten shipped to South Africa. That means there is a possibility for another round of recalls.

The FDA's top veterinarian, Stephen Sundlof, says finding melamine in so many products "would certainly lend credibility to the theory that it was maybe intentional."

Melamine, which is used to make plastics in the United States and as a fertilizer in Asia, contains nitrogen. Nitrogen can appear to boost the level of protein in products.

The revelations have led the FDA to expand the number of products it is testing as they enter the United States. So far, those inspections at the border have not turned up any melamine in wheat gluten. Tainted wheat gluten used by Menu Foods is suspected in sickening hundreds, if not thousands of pets.

Some of the tainted pet food has apparently made it into feed for hogs. Federal agencies are trying to determine if it was actually fed to animals and whether it may have reached the human food supply.