The head of an Ontario-based pet food company said Friday he has no idea how rat poison got into its products, but denied his company was negligent.

"At this stage, we don't know," Menu Foods chief executive Paul Henderson told reporters at a news conference in Mississauga, Ont. "We have a lot of work to do."

Henderson also offered an apology to pet owners across Canada and the U.S. who have lost their pets or been subject to worrying over their well-being.

"Our hearts and sympathies go out to them as they struggle with this," he said. "We understand what pet ownership is."

The company first noticed "alarm bells" after animals became ill in taste-testing of wet food products during the first week of March, Henderson said. The company immediately began testing the products and found no problems.

"We don't believe in the first place that our quality was lax to begin with," the company's executive vice-president Dr. Richard Shields said.

Earlier Friday, agricultural and veterinary officials in the U.S. said they had found the poison in pet food blamed for deaths of cats and dogs in North America.

New York State Agriculture Commissioner Patrick Hooker identified the chemical as aminopterin. Aminopterin can cause cancer, birth defects and kidney damage in dogs and cats, the department said.

Aminopterin is used as a rat poison in some countries, but is not registered for that purpose in Canada or the U.S.

Any amount of the chemical in pet food is too much, Hooker said.

The company has confirmed the deaths of 15 cats and one dog in the U.S. resulting from kidney failure. No deaths have been confirmed in Canada, although at least three Canadians have blamed the tainted food for their pets' deaths.

Menu Foods last week issued a North America-wide recall of 91 different types of dog and cat food manufactured between Dec. 3, 2006, and March 6, 2007.

Henderson also said there was "no reason to suspect" the problem goes beyond the products already recalled.

The company and investigators at the U.S. Food and Drug Administration had been focusing on wheat gluten, a source of protein in the pet food. While the ingredient does not cause kidney failure, it could be contaminated by heavy metals, mould toxins or poison, health officials said.

ABC News reported the chemical was on wheat imported from China.

"Right now, I couldn't speculate how it got in. That's wide open," Daniel Rice of the New York State Food Lab told a news conference. "If we can identify it as being in the gluten, which has not been confirmed yet, then that would help us with the trace-backs."

Wheat gluten from China under examination

The company will begin testing all of the suspect raw materials to identify how the substance entered its supply chain.

Henderson confirmed Friday that the company used wheat gluten from China as an ingredient in its products, but did not confirm whether that was the ingredient in question.

"We have identified a correlation from a single ingredient and we stopped using it," Henderson told reporters.

He added the two U.S. facilities where the tainted pet food was produced are still in operation and described the possibility of tampering as "remote."

Knowing the type of chemical should help veterinarians to treat animals sickened by the cuts-and-gravy style pet food.

Toronto lawyer Joel Rochon has filed a $60-million lawsuit against Menu Foods. Another national class-action lawsuit filed Thursday seeks compensation for pet owners who purchased the food.

The allegations in the lawsuits have not been proven in court. Menu Foods isn't commenting on the lawsuit.

"The issue of litigation, we will follow in due course," Henderson said.

The Menu Foods recall covers pet foods sold throughout North America under the Iams, Nutro and Eukanuba labels. In Canada, the recall also covers Loblaws President's Choice brand, as well as the house brands for Dominion, Sobeys and other supermarket chains.

With files from the Associated Press