Medical examiners tried to determine Friday why a 12-year-old girl died after collapsing at a wave pool in a Walt Disney World (search) water park — the third time in two months a child has died or become critically ill at the resort.

Lifeguards noticed Jerra Kirby (search), of Newport News, Va., on a ledge of the wave pool at Typhoon Lagoon on Thursday and asked if she was OK, sheriff's officials said. She said she was fine and wanted to be left alone, but passed out when she stood up, officials said.

Lifeguards performed CPR until paramedics arrived and took her to the hospital, where she was pronounced dead.

Results of Friday's autopsy could take several weeks if there is no obvious sign of trouble, said Steve Hanson, chief investigator for the Medical Examiner's Office (search) in Orlando. Relatives who were with her said she had no known medical problems, the sheriff's office said.

Disney spokesman Bill Warren said the park was operating normally Friday because "there was nothing mechanical wrong" with the pool, which simulates ocean waves.

Three other people have died this year at Disney World, out of the millions who visit the park each year. Two of the victims were adults in poor health; the case of a 4-year-old Pennsylvania boy who died while riding Epcot's "Mission: Space" in June remains under investigation.

In addition, a 16-year-old British girl suffered cardiac arrest last month after exiting "The Twilight Zone Tower of Terror" ride at the Disney-MGM Studios. She was still in critical condition Friday.

Theme park consultant Randy King, a former safety director at amusement park operator Six Flags, said he wasn't aware of safety problems with wave pools. "There's absolutely no telling what was wrong with her," he said.

The parking lot outside the Typhoon Lagoon park was full on Friday. Concilia Chitati, who was visiting from Britain, called the death "worrying" but not enough to prevent her from enjoying the wave pool.

"So far, our trip has been very nice," Chitati said.