The Food and Drug Administration said Friday that it has not found rat poison in pet food that has been killing animals, but that it has found melamine, a chemical commonly used to make plastic cutlery that is also used in fertilizer.

Hours after the announcement, the nationwide pet food recall, which had involved only so-called wet foods -- all manufactured by Menu Foods Inc. and sold under a variety of brand names -- was expanded to include one brand of dry cat food, Prescription Diet m/d Feline, made by Hills Pet Nutrition.

The brand was found to have been made with a batch of wheat gluten shipped to the United States from China that the FDA said was laced with melamine.

Scientists found melamine, which is used as a slow-release fertilizer in Asia, in the urine of cats sickened by the recalled pet foods made by Menu Foods, officials said at a news conference The recalled pet food has been blamed for at least 16 deaths of pets.

Officials said they are still tracing the distribution of the tainted wheat gluten to all the companies that had received it. Additionally, FDA officials said that they did not believe the contaminated wheat gluten had entered the human food supply, but that they were testing all wheat gluten imported from China for melamine.

The FDA's focus on melamine was contested by the New York State Food Laboratory, the testing facility that announced March 23 that it had identified aminopterin, a rat poison, in samples of recalled cat food made by Menu Foods.

"We don't think this is the final conclusion," said Jessica Chittenden, a spokeswoman for the New York State Department of Agriculture and Markets. "Melamine is not a known toxin. ... We are confident we found aminopterin, and it makes sense with the pathology."

Researchers at Cornell University in Ithaca, N.Y., also said they were unable to find evidence of the rat poison in the pet food samples they tested.

"There is little information in the scientific literature on melamine exposure on dogs and cats," Sundlof said. "So it's very difficult to determine a level that would be harmful or even lethal."

The lack of information about melamine's toxicity left some scientists who have worked with the chemical skeptical about its involvement in poisoning pets.

"It's not what I would consider to be a particularly toxic material," said Daniel Shelton, a scientist at the federal Department of Agriculture, who published a study on melamine's potential to be a slow-release fertilizer in 1997.

"We're being vigilant," said Sharon Curtis Granskog, assistant director of communications for the American Veterinary Medical Association, the largest professional organization for veterinarians in the United States. "We're listening to everything that comes out.

She said the most important thing is for owners to keep a close watch on pets for the first signs of renal failure, which include decreased appetite, vomiting, diarrhea, lethargy and increased thirst.

"If it's caught early," she said, "they're having some success with it. Hopefully, there will be a decrease, because these products are off the shelves."

At a news conference Friday, Paul Henderson, chief executive of Menu Foods, said he was confident that all the company's products made after March 6 were "safe and healthy."

Officials from Menu Foods said they were working with the company's insurance company to compensate owners for veterinary bills and, in some cases, the death of their pets.

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