Washington ? The pet food linked to the deaths of 16 animals has shown no signs of contamination, the manufacturer says, and the company cannot explain why the cats and dog developed acute kidney failure and died.

In an interview with The Associated Press, Paul Henderson, the chief executive officer and president of Menu Foods, said Wednesday the company was looking at a single ingredient. He wouldn’t identify it, but the Food and Drug Administration has said the investigation was focusing on wheat gluten.

“Our hypothesis is that it is that ingredient that in fact represents the highest probability as to the cause,” Henderson said. “But we have been unable to prove that through scientific information.”

The animal deaths have led to the recall of 60 million cans and pouches of pet food sold throughout North America.

“This is a puzzling and troubling experience for everybody within this organization,” Henderson said from company headquarters in Streetsville, Ontario. “It is extremely disheartening that we haven’t been able to find the causative agent.”

Menu Foods last week recalled “cuts and gravy” style dog and cat food. It includes food sold under store brands carried by Wal-Mart, Kroger, Safeway and other large retailers, as well as private labels like Iams, Nutro and Eukanuba.

The FDA has sent investigators to New Jersey and Kansas plants operated by Menu Foods.

A complete list of the recalled products along with product codes, descriptions and production dates was available from the Menu Foods Web site, www.menufoods.com/recall . The company also designated two phone numbers that pet owners could call for information – (866) 463-6738 and (866) 895-2708.