PHILADELPHIA (AP) _ A 500-pound antelope that may have been spooked by a truck backfire vaulted from its pen at the zoo and injured a wheelchair-bound visitor before it collapsed and died, officials said.

The visitor, Kathleen Daly, 39, of Philadelphia, suffered a broken leg Thursday and was listed in good condition at Presbyterian Hospital, authorities said.

Philadelphia Zoo spokeswoman Arlene Kut said the $10,000 sable antelope jumped an 8-foot gate and led zookeepers on a half-hour chase.

Visitors were asked to move into nearby buildings as the antelope raced around zoo paths, but Ms. Daly's friends apparently froze when they saw the animal rushing toward them, Ms. Kut said.

Unable to stop in time, the antelope skidded into the wheelchair, knocking the woman to the ground, Ms. Kut said.

''That location was in the vicinity of the lions, and we think the antelope may have become even more panicked when it caught their scent,'' Ms. Kut said.

Zoo workers cornered the bleeding animal at the African Plains exhibit, but it died about 10 minutes later, either from stress or from injuries received in the leap from the pen, officials said.

''We think it may have been frightened by a truck backfire,'' Ms. Kut said. ''Something must have frightened it very badly. Nothing ever escaped from that holding pen before.''

The animal had been at the zoo less than a month. The zoo planned to mate it with two males that had been ordered but not delivered.

''It's really a shame. She was really a beautiful animal and normally very calm,'' Ms. Kut said.