The Philadelphia Zoo's Petal - at 52, the oldest African elephant in a U.S. zoo - died this morning.

The animal staff was unable to determine the cause of death, but tests are being conducted and results are expected over the next few weeks.

Andrew Baker, vice president for animal programs, said the elephant care staff found Petal lying on her right side in her stall when they arrived shortly before 7 a.m.

They were instantly alarmed because she typically sleeps standing up.

The zoo's veterinarians and other animal care staff were immediately called. The team attending Petal tried for nearly two hours to comfort her and to help her to her feet.

But she died at approximately 9:15 a.m.

Petal's stall had a video monitor, and Baker said it appeared that Petal's right rear leg buckled suddenly earlier in the morning, and she collapsed.

"Petal had been in excellent health through her years at the zoo, showing no recent signs of illness or decline," Baker said in a zoo statement. "We will know much more in the days ahead."

Petal lived with two other African elephants, Kallie, 25, and Bette, 24. "Our elephant care staff will continue to monitor Kallie and Bette as they both had close bonds with Petal," Baker said.

Zoo CEO Vikram H. Dewan call Petal "a great ambassador for the endangered and threatened wild elephants of Africa and Asia."

He said in a statement that Petal "will be warmly remembered by staff and generations of zoo visitors as one of our most beloved animals. This is a great loss to the Philadelphia Zoo family and the greater Philadelphia community. . . . She will be missed."