Representatives of a food laboratory in Albany, N.Y., and the New York agriculture commissioner confirmed Friday afternoon that a substance used as a rat poison was found in pet food that is part of a massive recall.

After several pet deaths, Menu Foods recalled food sold under nearly 100 brand names.

The chemical was identified as aminopterin, a rodenticide that is also used in trials to treat leukemia.

Earlier, ABC News reported that the chemical was on wheat imported from China. However, at the news conference Commissioner Patrick Hooker and Donald Smith, dean of veterinary medicine at Cornell University, said that they could not confirm the source of the contamination.

Hooker also said that the toxin is not registered for legal use in the U.S. for rodent control.

They also said that the Food and Drug Administration and Menu Foods had been notified of the finding. Both stressed that the results were just one step in a process to determine how the food became contaminated.

Smith said the news conference was called to provide information to the public, as well as other researchers looking into the situation.

They said that they tested final products, not individual ingredients.

Sixteen animal deaths led to the recall of 60 million cans and pouches of pet food sold throughout North America under 95 brand names.