A zoo where a 2-year-old boy fell into an exhibit and was fatally mauled by African wild dogs had met or exceeded all safety standards for animals and visitors, proving that no exhibit is "fail-proof," the zoo's president said Monday.

Nearby staff responded "within seconds" on Sunday but quickly determined the dog attack was fatal and didn't send handlers into the enclosure to intervene, Pittsburgh Zoo and PPG Aquarium president Barbara Baker said.

Instead, the dogs were recalled into an indoor enclosure as they've been trained to respond, though four of the 11 lingered near the boy even after blank anesthetic darts, used out of an abundance of caution for the boy's safety, were fired to shoo them away. One of the dogs, which are endangered, was fatally shot by police.

Baker said the zoo has been open since 1898 and this is the first time there's been "a visitor incident of this magnitude." She called the boy's death a "horrible, horrible tragedy" and said there's "no such thing as a fail-proof exhibit."

Baker struggled to maintain her composure during her Monday news conference and made clear she was careful to consider the family's feelings before answering questions, including one about how the boy died.

She paused several seconds before saying, "I'm trying to think of a family-sensitive way to address that. The child did not die from the fall. The child was mauled by the dogs."

Police were investigating, though police Cmdr. Thomas Stangrecki, who attended Baker's news conference, said he was there only to observe.

The boy's mother had put him on a wooden railing at the edge of a viewing deck before he fell late Sunday morning. He bounced out of netting below before dropping more than 10 feet into the dogs' enclosure.

Baker said the Allegheny County medical examiner's office determined the boy survived the plunge. The medical examiner's office has not publicly confirmed its findings or released the boy's name. Pittsburgh media outlets reported the boy was from suburban Whitehall, but The Associated Press couldn't immediately confirm that.

A spokesman for the Association of Zoos and Aquariums, which accredits zoos and aquariums that meet certain standards, said the Pittsburgh Zoo successfully completed its five-year review in September. And Baker said the U.S. Department of Agriculture had inspected the zoo and found it safe in recent months.

The wild dogs, about as big as medium-size domestic dogs, resemble wolves. They hunt in packs, targeting prey such as antelopes, gazelles and wildebeest calves, and their kills are noted for their savagery. Visitors view the zoo's dogs from a wooden deck that's enclosed except for the front, where the roughly 4-foot-high wooden railing is located.

The exhibit is closed indefinitely, and the dogs have been quarantined, though Baker said they will not be euthanized. The zoo also has been closed since the boy's death but will reopen Tuesday, Baker said.

Mourners left teddy bears and other items outside the zoo and responded to a condolence message on the zoo's Facebook page.

Nearly 1,000 people commented on the post, some passionately condemning the boy's mother and others urging compassion and understanding. More than a few parents acknowledged lifting their children onto the rail, too.

Baker said zoo officials "discourage" parents from setting their children on the wide, wooden railing, which slopes toward the viewing platform at a 45-degree angle so a child placed there would be more likely to fall backward into a parent's arms than forward into the exhibit.

Nate Legler takes his 4-year-old daughter to the Pittsburgh zoo two or three times a month and said it's safe and there's no way for a small child to climb over the wooden railing without assistance. He said he has lifted his daughter to give her a better view but never close enough to the railing to be dangerous.

"You can take your child to the Grand Canyon and hold them up to let them look down but not feel unsafe like you're going to drop them into the canyon," he said.

The director of animal care at the Columbus Zoo and Aquarium in neighboring Ohio, Doug Warmolts, said exhibits are designed with visitors' expectations in mind because they all want to have unobstructed views and "the up-close experience" but the first priority is always safety.

He said accidents like the boy's death in Pittsburgh are "one of those things that keep you up at night."

"I'm sure it's going to send ripples through our industry and everybody's going to double-check their measures and how to respond to things like that," he said.

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Associated Press writers Michael Rubinkam in Allentown and Kevin Begos in Pittsburgh contributed to this report.