San Francisco Zoo Director Manuel A. Mollinedo also admitted that it is becoming increasingly clear the 300-pound Siberian tiger leaped or climbed out of its open-air enclosure, perhaps by grabbing onto a ledge.
"She had to have jumped," he said. "How she was able to jump that high is amazing to me." Mollinedo said investigators have ruled out the theory the tiger escaped through a door behind the exhibit.
According to the Association of Zoos & Aquariums, the walls around a tiger exhibit should be at least 16.4 feet high. But Mollinedo said the nearly 70-year-old wall at the zoo's big-cat enclosure was 12 feet, 5 inches, with a moat 33 feet across.
He said safety inspectors had examined the 1940 wall and never raised any red flags about its size.
"When the AZA came out and inspected our zoo three years ago, they never noted that as a deficiency," he said. said. "Obviously now that something's happened, we're going to be revisiting the actual height."
On Wednesday, Mollinedo said that the wall was 18 feet high and the moat was 20 feet wide. Based on those earlier, incorrect estimates, animal experts expressed disbelief that a tiger in captivity could have made such a spectacular leap.
AZA spokesman Steven Feldman said that the minimum height is just a guideline and that a zoo could still be deemed safe even if its wall were lower.
Accreditation standards require "that the barriers be adequate to keep the animals and people apart from each other," Feldman said. "Obviously something happened to cause that not to be the case in this incident."
Many other U.S. zoos have significantly higher walls around their tigers.
Feldman would not comment on how difficult it would be for a tiger to scale a 12½-wall. But Siberian tigers are known to have phenomenal strength, at least in the wild.
"There are rare glimpses of this in the real world that suggest, when taunted, tigers can be fairly extraordinary in their physical feats," said Ronald Tilson, who is director of conservation at the Minnesota Zoo and the big-cat expert who sets safety standards for tiger exhibits at North American zoos.
The animal went on a rampage near closing time on Christmas Day, mauling three visitors before it was shot to death by police. Carlos Sousa Jr., 17, died and two brothers, ages 19 and 23, suffered severe bite and claw wounds.
Police are still investigating and have declared the big-cat exhibit a crime scene.
Investigators are trying to determine if one of the tiger's victims climbed over a railing around the enclosure, reports CBS News correspondent John Blackstone.
"There is a shoe print on the railing," said San Francisco police chief Heather Fong.
The San Francisco Chronicle, citing anonymous sources, reported Thursday that police are looking into the possibility that the victims may have dangled a leg or other body part over the edge of the moat. The newspaper said police had found a shoe and blood inside the enclosure.
But at an afternoon news conference, Fong said police had no information that anyone had put a leg over the railing, and she said no shoe was found in the animal's enclosure. She did not address whether the victims had teased the tiger.
She said a shoeprint was found on the railing of the fence surrounding the enclosure, and police are checking it against the shoes of the three victims.
"Right now, what I want to know is if it was taunting, who did it? Why, why wasn't this protected right? I want some answers," said the dead teenager's father. As for the zoo, "They know what they did wrong, they know what they did."
Mollinedo said surveillance cameras and new fencing will be installed around the exhibit. The zoo will remain closed Friday.
At the Bronx Zoo, the tigers are surrounded by a 20-foot-high chain-link fence with a 5-foot overhang that curls inward at the top. An electrified wire runs along the inside of the fence.
The Philadelphia Zoo said it has 16-foot walls topped with a 3-foot overhang. At the Virginia Zoo in Norfolk, Va., the walls are 15 to 20 feet high with a 5-foot overhang and an electrified wire. At the Reid Park Zoo in Tucson, Ariz., the wire fence is about 17 feet.
At the Columbus Zoo and Aquarium, Assistant Director Don Winstel said he checked the architectural drawings and plans for the enclosure on Wednesday, and found that the walls and fence around the tigers are no lower than 16 feet.
But "now that you mention it, I think I'll take a tape measure out there tomorrow and make sure," he said.
The AZA said in a statement that this was the first time a visitor had been killed because of an animal escape at an AZA-accredited zoo.
"The San Francisco Zoo is a great zoo, it's an accredited AZA member in good standing, and it has our support during this difficult time," AZA president and chief executive Jim Maddy said.