Dec. 25, 2007: In this video image from KGO television, paramedics arrive at San Francisco General Hospital with one of the victims of a tiger attack.
Dec. 25: A mauling victim at the San Francisco Zoo is assisted by paramedics.
The San Francisco Zoo was closed to visitors Wednesday as police investigated a tiger attack they say may have been provoked by visitors' taunting the animal, leaving one man dead and two brothers injured.
Police searched the zoo to ensure there were no others injured and no more animals out of their cages after the Christmas day attack in which the 300-pound tiger, Tatiana, escaped its cage.
One witness said at least one of the victims had provoked the tiger, which had been out of its cage an estimated 15 to 20 minutes, police said. Tatiana also ripped the flesh off a zookeeper's arm just before Christmas 2006.
When officers arrived after responding to the 911 call, they "saw a tiger sitting next to a person who was sitting on the ground," Police Chief Heather Fong said.
They then "yelled at the animal to stop. They did not fire immediately. ... when the yelling was occurring the animal turned toward the officers" and that’s when the officers shot the tiger, she said.
Fong said officers conducted four searches of the zoo after the attack.
"We are confident there are no additional victims," she said.
Fong said the department has opened a criminal investigation to "determine if there was human involvement in the tiger getting out or if the tiger was able to get out on its own."
Police said they have not ruled anything out, including whether the escape was the result of carelessness or a deliberate act.
Fong said officers were gathering evidence from the tiger's enclosure as well as accounts from witnesses and others.
One zoo official insisted the tiger did not get out through an open door and must have climbed or leaped out. But Jack Hanna, former director of the Columbus Zoo and a frequent guest on TV, said such a leap would be an unbelievable feat, and "virtually impossible."
"There's something going on here. It just doesn't feel right to me," he said. "It just doesn't add up to me."
Instead, he speculated that visitors might have been fooling around and might have taunted the animal and perhaps even helped it get out by, say, putting a board in the moat.
Zoo Director Manuel Mollinedo said the zoo planned to bring in outside experts to evaluate the exhibits' safety and conduct thorough analysis of existing outdoor cat exhibits … to ensure animal safety and public safety."
Mollinedo said he hoped to have the zoo open on Thursday.
The three men — one of them 19 years old and the others in their early 20s — were attacked just after 5 p.m. Tuesday on the east end of the 125-acre zoo grounds near Ocean Beach, police spokesman Steve Mannina said.
The two survivors were upgraded from serious to stable condition Wednesday afternoon.
"They’re in good spirits. They looked absolutely fantastic," said Dr. Rochelle Vicker of San Francisco General Hospital.
She added they were being treated with antibiotics and monitored for infections from their claw and tooth wounds to their heads, necks, arms and hands.
They suffered "pretty aggressive bite marks," police spokesman Steve Mannina said.
"Wild animals have about seven times the strength per pound than humans do," animal behavior specialist Diana Guerrero told FOX News.
"The caging or enclosures are the territories of captive zoo animals. ... They’re hardwired for certain behaviors."
When they escape their enclosures, however, that behavior gets thrown off kilter and anyone nearby must remain calm and quiet and out of their way to avoid agitating them.
Zoologist Ron Magill agreed, saying that wild animals in captivity lose their fear of humans and will "take advantage of any possibility" to escape.
"You can take the animal out of the wild; you cannot take the wild out of the animal," he said.
The zoo's director of animal care and conservation, Robert Jenkins, could not explain how Tatiana escaped. The tiger's enclosure is surrounded by a 15-foot-wide moat and 20-foot-high walls, and the big cat did not leave through an open door, he said.
"There was no way out through the door," Jenkins said. "The animal appears to have climbed or otherwise leaped out of the enclosure."
The first attack happened right outside the Siberian's enclosure — the victim died at the scene. A group of four officers came across his body when they entered the dark zoo grounds, Mannina said.
The second victim was about 300 yards away, in front of the Terrace Cafe. The man was sitting on the ground, blood running from gashes in his head and Tatiana sitting next to him.
The cat attacked the man again, Mannina said. The officers approached the tiger with their handguns. Tatiana moved in their direction and several of the officers fired, killing the animal.
Only then did they see the third victim, who had also been mauled.
Although no new visitors were let in after 5 p.m. Tuesday, the grounds had not been not scheduled to close until an hour later, and 20 to 25 people were still in the zoo when the attacks happened, zoo officials said. Employees and visitors were told to take shelter when zoo officials learned of the attacks.
"This is a tragic event for San Francisco," Fire Department spokesman Lt. Ken Smith said. "We pride ourselves in our zoo, and we pride ourselves in tourists coming and looking at our city."
There were five tigers at the zoo — three Sumatrans and two Siberians. Officials initially worried that four tigers had escaped, but soon learned only Tatiana had escaped, Mannina said.
On Dec. 22, 2006, Tatiana reached through the bars of her cage and grabbed a keeper, biting and mauling one of the woman's arms and causing deep lacerations. The zoo's Lion House was temporarily closed during an investigation.
California's Division of Occupational Safety and Health blamed the zoo for the assault and imposed an $18,000 penalty. A medical claim filed against the city by the keeper was denied.
Last February, a 140-pound jaguar named Jorge killed a zookeeper at the Denver Zoo before being fatally shot. Zoo officials said later that the zookeeper had violated rules by opening the door to the animal's cage.
After last year's attack, the zoo added customized steel mesh over the bars, built in a feeding chute and increased the distance between the public and the cats.
Tatiana arrived at the San Francisco Zoo from the Denver Zoo a few years ago, with zoo officials hoping she would mate with a male tiger. Siberian tigers are classified as endangered and there are more than 600 of the animals living in captivity worldwide.
The Associated Press contributed to this report.