Petal, who was 52 years old, did not appear to have died of heat-related causes, and an initial exam revealed no health problems, said Andrew Baker, vice president for animal programs at the Philadelphia Zoo.

The Philadelphia Inquirer reported tests were being conducted and results are expected over the next few weeks.

Baker said in the wild, African elephants such as Petal typically live to their early or mid-40s.

Petal was found Saturday lying on her right side in her stall shortly before 7 a.m. She stopped breathing and died about 9:15 a.m., the newspaper reported.

After viewing video tape of Petal's stall, Baker said it appeared her right rear leg had buckled suddenly earlier in the morning and she had collapsed.

Beyond that, "we don't have any viable theories," Baker said.