But an animal-rights group believes it might have been more than old age that killed her - a claim dismissed by federal officials.

In Defense of Animals, based in San Rafael, Calif., filed a complaint earlier this week with the U.S. Department of Agriculture calling for an investigation into Petal's death.

"There's a lot of unanswered questions," said Catherine Doyle, the group's campaign director. "We want the USDA to review the surveillance video and determine if Petal and the other elephants were being adequately monitored and provided with proper care."

The USDA has already completed its investigation.

"We have looked into it," said Karen Eggert, a USDA spokeswoman. "We found no signs of noncompliance or any issues with the welfare of the animal."

Zoo authorities still don't know what caused Petal's unexpected death.

"It may take several more weeks before the postmortem is concluded," zoo spokesman Bill Larson said yesterday.

Petal appeared to be in very good condition, especially for an elephant her age, Larson said. She showed no recent signs of illness or decline.

"At 52, Petal lived longer than available information indicates most female African elephants do in the wild," Larson said. He put the life span of wild elephants at the early to mid-40s.

Doyle said that although elephants in captivity don't have to face drought, famine or poachers, their life spans are significantly shorter than those of their wild cousins.

African elephants can live up to 70 years in their natural habitat, Doyle said. Elephants in captivity live an average of 33 years, she said.

"In Africa, there are females breeding into their 50s and 60s," Doyle said. "One very famous elephant had a calf in her 60s."

Doyle contends that elephants are not suited to life in urban zoos.

"In the wild, these animals are active 20 hours a day," Doyle said. "They're always on the move. In Philadelphia, the elephants were reported chained overnight."

In a statement, Larson said that Petal was never chained overnight and that the USDA was informed within hours of her death.

By prodding the USDA, Doyle said she hoped the zoo would eventually provide access to the animal's necropsy and pathology report.

"We feel the public has a right to know," she said.