Two red pandas at the National Zoo died after eating tiny pieces of toxic gas-producing pellets that apparently were left on the ground when the substance was buried in rat burrows in their yard, the zoo's pathologist said yesterday.

Both animals ingested fragments of the pellets, which then formed a toxic gas in their stomachs, according to detailed toxicological studies, pathologist Richard J. Montali said in an interview.

Mothball-size pellets of aluminum phosphide, which mixes with groundwater to form poisonous phosphine gas, were buried in the red panda yard late in the afternoon of Jan. 10, part of an effort to combat the zoo's rat population. The pandas, an endangered species, were found dead the next morning.

"We found material in their stomachs that formed phosphine gas," Montali said. "This suggests they actually ingested" pieces of the pellets, rather than inhaling the gas as initially suspected. The main toxicology analysis was done by Michigan State University, although the zoo also sent tissue samples to other laboratories, he said.

The red pandas' deaths and the more recent demise of a pygmy hippopotamus are the latest in a string of animal deaths that have led to zoo and Smithsonian Institution investigations. The American Zoo and Aquarium Association also is looking at the recent animal deaths while processing the zoo's reaccreditation application, which comes up every five years.

In the aftermath of the deaths, zoo Director Lucy H. Spelman acknowledged serious judgment errors and communication problems and ordered an overhaul of animal care and pesticide control procedures. She also ousted two senior zoo employees, reassigned another and halted the zoo's use of Piedmont Pest Control, a private firm whose owner had buried the pellets in the yard as part of the rat abatement program.

Montali, who is preparing a written report on the red panda deaths, said the toxic gas in the animals "basically knocked out their enzyme systems." He said the gas-producing fragments they ingested probably were left on the ground when the pesticide contractor buried the pellets in the rat burrows.

The pathologist said there was no evidence that the animals dug up the pellets, which were buried two feet deep in the rat burrows and covered with metal plates.

Noting the curious and "chemo-sensory" tasting and smelling nature of red pandas, Montali said, "When the contractor left, they probably came down and looked around and found the fragments."

Montali said Piedmont's owner, Jarrett L. Cross, told him the day the pandas were found dead that the pesticide product he used "had been on the shelves" for a while.

The fragments probably broke off, Montali said. "They were very small, and my thinking is that the contractor and his two helpers probably didn't even notice," Montali said.

Cross, who operates out of his Montgomery County home and had done pesticide work for the zoo since 1999, has declined repeated requests for comment on the red panda incident. He did not return calls yesterday.

A key concern is that the decision to bury the pellets in the red panda yard was made without consulting a zoo veterinarian or the pathology staff.

Both red pandas, ages 71/2 and 51/2, were otherwise healthy, Montali said.

Red pandas often live into their teens while in the care of zoos.

"What happened is shattering to all of us," said Montali, whom Spelman has now put in charge of pest control operations. Had plans to use the poison gas "come by me, I would have totally dismissed it as something that should never be done."

Montali said he has formed a task force of Smithsonian pest control experts and others who are developing an integrated pest management program for the zoo, which in the past year has struggled with a worsening rat problem. He said he also met with curatorial staff on pest control and safety and will present his report on the red panda deaths at a meeting with the zoo's veterinarians.

"We need to show the public and our own people that we take what happened very seriously," he said.