The accident occurred on a glorious, postcard-ready summer afternoon in New York, free of the usual culprits of lightning or snow. A large, healthy tree branch, 30 feet above a well-traveled path just outside the Central Park Zoo, snapped, fell and killed a 6-month-old girl as horrified visitors looked on.

The accident, which occurred in one of the park’s most popular locales, could be viewed as a freak occurrence. But it is also the latest in a string of deadly episodes that have plagued the park in the past year, all involving tree branches that abruptly plummeted to earth, killing or seriously injuring passers-by.

It has been years since New Yorkers wondered whether Central Park is safe. But the alarming frequency of the accidents has turned a spotlight on the trees, and whether the entities that oversee the park are doing enough to safeguard the public.

On Sunday, the city’s parks department said it did not know why the branch had fallen, killing the 6-month-old, Gianna Riccuitti, and critically injuring her mother, Karla DelGallo, 33. Furthermore, the city acknowledged that it did not know who was responsible for the upkeep of the tree.

“The investigation as to why the limb fell is ongoing, as is a review of specific responsibility for tree maintenance,” Vickie Karp, a parks spokeswoman, said in a statement.

Generally, the park’s 26,000 trees are overseen by the Central Park Conservancy, the private nonprofit group that has maintained nearly full oversight of the park’s operations and horticulture since 1998, under a formal agreement with the city.

But officials at the conservancy, which counts many of the city’s political and financial elite as members, would not say on Sunday whether the tree was under their purview or if they were conducting their own investigation.

The group referred questions to the police or the zoo’s operator, the Wildlife Conservation Society, which is considered by the city to be responsible for the maintenance and operation of the zoo and its environs.

But the society would not say on Sunday whether it was responsible for the tree. Reached by telephone, a spokesman, Max Pulsinelli, said he could not comment beyond a brief statement expressing condolences for the accident’s victims. Ms. DelGallo remained in critical but stable condition at NewYork-Presbyterian Hospital/Weill Cornell Medical Center on Sunday.

Park officials have said that it is impossible to ensure the safety of every single tree in the park, and the conservancy spends upward of half a million dollars annually on tree maintenance.

“It should be minimized,” said Henry J. Stern, a former city parks commissioner. “It is impossible to prevent this completely.”

Still, the question of how closely the trees are tracked has now been raised in the courts. A lawsuit, filed by the family of a Brooklyn man who was killed by a falling branch in February, claims that the conservancy was negligent in removing the tree, which stood at the east side of the park near 69th Street.

The family’s lawyer, Alan M. Shapey, said the tree had been deemed dangerous by the conservancy in December 2009 and had been given a high priority for removal. But the American elm stood untreated for two more months, until the day of the death of the man, Elmaz Qyra, according to the lawsuit. Officials at the conservancy did not respond on Sunday to those claims.

“Why would that tree be there two and a half months after it should have been removed?” Mr. Shapey said in an interview. “Are they waiting for funds? For equipment?”

“I can’t be walking through Central Park looking up at the trees, wondering which one is going to hit me,” he added. “Do I have to wear a hardhat?”

A lawsuit is also pending in the case of Sasha J. Blair-Goldensohn, 33, a Google employee who was struck in the head in July by a rotting four-inch-thick branch on the west side of the park. He survived but suffered from brain and spinal damage.

On Sunday, the mood on the zoo’s promenade was nearly back to normal. Many parkgoers did not appear to notice the broken tree limb above the site of Saturday’s accident.

“The other trees should be roped off,” said Gary Frumberg, 72, who was seated on a nearby bench. “You would think there would be a more concerted effort to inspect all the branches.”

Mr. Frumberg, who said he often brought his grandchildren to the zoo, said he was now nervous about making future visits. “When I come here now, I’m going to be looking up at the trees,” he said.

Johnnie Lacend, 51, said the accident would not keep him from making the trip to the zoo with his daughter and grandson. But the death “did freak us out,” he said.

Mr. Stern, who served as parks commissioner for 16 years, said he could not recall three such accidents in Central Park in a single year.

Recalling his own stewardship of the park, Mr. Stern said its trees could be an unpredictable bunch.

“The trees that you think are the problem trees are not the ones that fall,” he said. “It’s not predictable. When there was an accident, it was some tree we had never thought about.”

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