Spokeswoman Jessica Chittenden would not identify the chemical or its source beyond saying it was a rodent poison.
State agriculture officials and Cornell University's Animal Health Diagnostic Center scheduled a news conference Friday afternoon to release laboratory findings from tests on the pet food conducted this week.
Neither agency would identify the poison, but one report identified it as aminopterin, which inhibits the growth of malignant cells and is used as a cancer drug. It also suppresses the immune system. In high doses, the chemical is highly toxic.
It is used to kill rats in some countries but is not registered for that use in the United States, according to the Environmental Protection Agency.
The deaths led to a recall of 60 million cans and pouches of pet food produced by Menu Foods and sold throughout North America under 95 brand names. There have been reports of kidney failure, some fatal, in pets that ate the recalled brands. The company has confirmed the deaths of 15 cats and one dog.
Menu Foods last week recalled "cuts and gravy" style dog and cat food. The recall sparked concern among pet owners across North America. It includes food sold under store brands carried by Wal-Mart, Kroger, Safeway and other large retailers, as well as private labels such as Iams, Nutro and Eukanuba.
A complete list of the recalled products along with product codes, descriptions and production dates was available from the Menu Foods Web site.
The recall has led to at least three lawsuits against Menu Foods from pet owners who allege their animals got sick or died after eating recalled food.
"To find out they knew about this weeks ago, and that the cats they tested died!" former cat owner Dawn Marjerczyk told CBS. Marjerczyk's cat died after eating some of the tainted cat food. "Why wasn't it pulled off then? Why do so many people have to suffer right now?"
The company's chief executive and president said Menu Foods delayed announcing the recall until it could confirm that the animals had eaten its product before dying. Two earlier complaints from consumers whose cats had died involved animals that lived outside or had access to a garage, which left open the possibility they had been poisoned by something other than contaminated food, he said.
Wheat gluten itself would not cause kidney failure, but the common ingredient could have been contaminated by heavy metals or mold toxins, the FDA said.
"It's a loss that goes beyond belief," agreed Frederick Bobb, of Merrick, N.Y., whose 2-year-old bull mastiff, Princess, died of kidney failure on March 10. Bobb said his dog had eaten Nutro, one of the recalled dog food brands, all her life.