The bad news for animal lovers Friday was that an industrial chemical was found in recalled pet food, but the worst news was that authorities still didn’t know why hundreds of dogs and cats in North America fell ill or died.

The Food and Drug Administration said its tests of pet food made by Menu Foods Income Fund of Ontario, Canada, turned up melamine, a chemical used to make plastic, glue, fertilizer and paint. Scientists at Cornell University said they discovered the substance in the urine of sick cats and in the kidney of a cat that died after consuming some of the recalled food.

But it remained unclear exactly what caused pet illnesses and deaths that sparked this month’s sweeping recall.

Last week, scientists at the New York State Food Laboratory and the New York State Animal Health Diagnostic Center at Cornell identified aminopterin, a substance found in rat poison, as the possible culprit. But the federal testing did not confirm the presence of aminopterin.