Federal investigators said a chemical used to make plastics, not rat poison, lead to a deadly contamination of pet food.

The Food and Drug Administration said Friday that a substance called melamine, used in fertilizer, somehow made its way into cans of a variety of wet pet foods that contained gravy.

Thousands of pet owners reported their animals were sickened by the food, leading to a recall of 60 million food packages.

Further, FDA officials said even more Menu Food products may have been contaminated than previously thought.

Officials said one type of dry pet food may also be contaminated, but, despite pressure, they would not say which brand and did not issue a recall.

Now, there is new controversy over how the federal government is dealing with the pet food problem.

FDA officials said they will not recall that dry food unless testing shows proof of contamination.

Stephen Sundlof, an FDA spokesperson, said, "(Contamination) is possible, but we're following every lead that we can."

Bruce Friedrich, of the group People for the Ethical Treatment of Animals, called the FDA's actions "morally reprehensible."

"The FDA can't be trusted to guard our companion animals," Friedrich said.

Tracey Powell, a pet owner, said for the first time in four years she's not sure what to feed her dogs for fear of contamination.

"They're part of the family," Powell said.

The FDA said the melamine-contaminated ingredients were supplied by China and that same supply is used by one dry food manufacturer that the FDA won't name.

The agency said so far, it has received at least 8,000 complaints from pet owners who said their pets have been sickened and confirmed at least 14 animals have died as a result of the contamination.

While there is no way to visually identify contaminated food, Menu Foods said all of its products made after March 6 are "safe and healthy."

For more about the recall, please click here.