Investigators have found rat poison in the pet food suspected of killing 15 cats and two dogs, but they can't explain how it got there. Meanwhile, veterinarians and federal regulators predicted that many more pet deaths and illnesses will be linked to the 95 brands of wet pet food voluntarily recalled March 10 by its manufacturer.

"I think we're going to see hundreds if not thousands of cases," said Brad Smith, director of the veterinary teaching hospital at UC Davis. "It's going to take some time to sort this out."

Menu Foods, the Canadian-owned pet food maker, expanded its recall Friday to all cans and pouches of the recalled brands of "cuts and gravy" foods regardless of when they were manufactured. The first recall was limited to pet foods made after Dec. 3.

The discovery of rat poison in pet food was announced Friday by New York state food safety experts who analyzed samples of the commercial pet food provided by the manufacturer.

The substance, aminopterin, is not licensed for use as a rat poison in the United States, but it is used for that purpose in other counties.

The drug is used in cancer research, and it formerly was used to induce abortions in the United States, scientists said.