The Food and Drug Administration said it found melamine in samples of the Menu Foods pet food, as well as in wheat gluten used as an ingredient. Cornell University scientists also have found the chemical, also used as a fertilizer, in the urine and kidney of a sick cat.
However, officials would not say if the melamine was the cause of the deaths and illnesses.
Menu Foods recalled 60 million containers of cat and dog food earlier this month after animals died of kidney failure after eating the Canadian company's products. It is not clear how many pets may have been poisoned by the apparently contaminated food, although anecdotal reports suggest hundreds if not thousands have died. The FDA alone has received more than 8,000 complaints.
Melamine can be found in kitchen utensils and countertops.
Meanwhile, animal rights advocates are calling on federal food safety regulators and pet food companies to expand a nationwide recall of dog and cat food to include dry varieties, claiming it also makes pets sick.
The Food and Drug Administration said Thursday it had no plans to suggest a wider recall to pet food companies, and veterinarians said they have not seen a trend of animals becoming ill after eating dry pet food.
The People for the Ethical Treatment of Animals plans to make the appeal Friday in Washington after it said it received complaints from pet owners who claim their animals suffered kidney failure after eating dry pet food.
Norfolk, Va.-based PETA wants the FDA and the companies to extend the recall to foods that have received complaints, chemically test it and perform necropsies on the animals involved. It also wants companies prosecuted if the FDA's probe turns up wrongdoing.
FDA spokeswoman Julie Zawisza said she did not know how many of the complaints the agency has received have concerned dry pet food.
Nearly 100 brands of pet food were recalled after animals suffered kidney failure. The recall involved "cuts and gravy" style dog and cat food. The recall covered products carrying names of major brands including Iams, Nutro and Eukanuba.
New York state's food laboratory last week identified aminopterin as the likely culprit in the recall. Aminopterin is a derivative of folic acid that was once used to induce abortions and is now banned as a rodent poison in the United States. It can cause kidney damage in dogs and cats.
The lab later also detected melamine in the food.
Veterinarians aren't seeing a trend of pets getting sick off dry food, said Paul Pion, founder of the Veterinarian Information Network. He said since so many people use dry food, you would expect to see many more ill pets if the food was tainted.
"I wouldn't put much credence in it, but it's not out of the realm of possibility," Pion said.
The Veterinary Information Network reported Tuesday that at least 471 cases of pet kidney failure have been reported since the recall, and more than 100 pets have died. Menu Foods has confirmed 16 pet deaths.
Pion, a California veterinarian, said only 10 percent to 20 percent of the people who belong to his Web site had responded to a request for information.
"If we're only getting 10 percent of the veterinarians, you can do the math," he said.
CBS News Early Show resident veterinarian Dr. Debbye Turner said Wednesday, "Pretty much every vet you talk with has one or two cases."
She adds that another Web site, PetConnection.com declares nearly 2,000 pets have died from eating the recalled food.
So who is right? In short, Turner says, nobody really knows, and, "We don't know that some of the pets reported on PetConnection aren't some of the same pets reported on VIN or reported in other series."