A two-year-old boy visiting the Pittsburgh zoo was killed yesterday morning when he fell off a railing that his mother had put him on top of to view a pack of African painted dogs, who pounced on the child and mauled him, police said.

It was not clear whether the boy died from the fall into the wild dog exhibit area or from the attack, said Barbara Baker, president and CEO of the Pittsburgh Zoo & PPG Aquarium.

"It was very horrific," said Lt. Kevin Kraus of the Pittsburgh police, who added that the dogs attacked "immediately" after the boy fell at about 11:45 am. Zoo officials at first estimated the boy fell 14 feet, but police said it was 11. It was not clear which is correct.

When the boy fell, other visitors immediately told staff members, who responded along with Pittsburgh police. Zookeepers called off some of the dogs, and seven of them immediately went to a back building. Three more eventually were drawn away from the child, but the last dog was aggressive and police had to shoot the animal, officials say.

The zoo was immediately closed; it was not clear when it would be reopened, authorities said.

Authorities didn't immediately release the name of the boy or his mother, but say she is 34 years old and lives in Pleasant Hills, just outside Pittsburgh. The boy's father arrived on the scene soon after the accident, police said.

The dogs are about as big as medium-sized domestic dogs, 2 to 2½ feet high and 37 to 80 pounds, according to the zoo. African wild dogs are also known as cape hunting dogs, spotted dogs, and painted wolves. They have large, rounded ears and dark brown circles around their eyes and are considered endangered.

The dogs hunt in packs in the wild, and Baker said that they "were in pack mentality" during the attack.

The dogs normally live in a 1.5 acre exhibit called the Painted Dog Bush Camp that's part of a larger open area called the African Savanna, where elephants, lions and other animals can be seen. Visitors walk onto a deck that is glassed on the sides, but open in front where the railing is located. Visitors can look out at the dogs below.

Zoo officials said there is also a mesh barrier beyond the railing, but Kraus said it was designed to catch small objects such as cameras, and not humans.

Steve Feldman, a spokesman for the Association of Zoos and Aquariums, said no one he's spoken to can recall any deaths of children at an accredited zoo over the last 40 years or more. Feldman said the Pittsburgh Zoo successfully completed its five-year review in September, which means it meets or exceeds all safety standards.

In May, some of the dogs crawled under a fence and escaped into a part of the exhibit that's usually closed. The zoo was on lockdown for about an hour as a precaution.

Past fatal attacks at zoos have prompted zoos around the nation to review safety features of their exhibits. In 2007, a tiger jumped over a wall at the San Francisco zoo, killing one visitor and wounding two others. In September a man jumped off an elevated viewing train at the Bronx Zoo in New York and was severely mauled by tigers.

Baker said the Pittsburgh Zoo has never had a visitor death. She said no decision has been made yet on the future of the exhibit.