Rat poison was found in tainted pet food that sparked a nationwide recall this month.
The Food and Drug Administration seeks to determine how it got in the food supply of Menu Foods.
Test results showed aminopterin, a type of rat poison, could be to blame for the massive recall in the United States.
"We have no idea how it could have gotten into the food product," said lab technician Dan Rice.
While the toxin is used to kill rats in other countries, it is only legal to use as a cancer drug in the U.S.
"There are any number of ways to contaminate food," said New York Agriculture Commissioner Patrick Hooker.
The FDA has focused on wheat gluten in the pet food that's suspected of causing kidney failure in dogs and cats across the country, killing at least 16, including 15-month-old Abby.
"To see her deteriorate was very hard," said pet owner Pat Sims. "In the end I literally carried her in to the vet."
Those deaths led to the recall of 60 million cans and pouches of pet food made by Menu Foods under more than 95 brand names.
"It's hard," said Sims. "We do feel guilty, because we trusted. You just trust it's going to be safe."
Even though pet owners and veterinarians may know what caused the contamination, they must wait for answers about how it happened.
Menu Foods' Web site has a complete list of all recalled products along with product codes, descriptions and production dates.
