SAN FRANCISCO - The big-cat exhibit at the San Francisco Zoo was cordoned off as a crime scene yesterday as investigators tried to determine whether a 300-pound Siberian tiger that killed a visitor escaped from its high-walled pen on its own or got help from someone, inadvertent or otherwise.

Police shot the animal to death after a Christmas Day rampage that began when the tiger escaped from an enclosure surrounded by what zoo officials said are an 18-foot wall and a 20-foot moat. Two brothers who also were visiting the zoo were severely mauled.

Police Chief Heather 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.

Similarly, Ron Magill, a spokesman at the Miami Metro Zoo, said it is unlikely a zoo tiger could make such a leap, even with a running start.

"Captive tigers aren't nearly in the kind of shape that wild tigers have to be in to survive," he said. Taunting can make an animal more aggressive, he said, but "whether it makes it more likely to get out of an exhibit is purely speculative."

The police chief would not comment on whether the animal was taunted.

The same tiger, a 4-year-old female named Tatiana, ripped the flesh off a zookeeper's arm just before Christmas a year ago while the woman was feeding the animal through the bars. A state investigation faulted the zoo, which installed better equipment at the Lion House, where the big cats are kept.

Zoo director Manuel Mollinedo said yesterday that he had given no thought to destroying Tatiana after the 2006 incident, because "the tiger was acting as a normal tiger does." As for whether Tatiana showed any warning signs before Tuesday's attack, Mollinedo said: "She seemed to be very well-adjusted into that exhibit."

It was unclear how long the tiger had been loose before it was killed. The three visitors were attacked around closing time Tuesday on the 125-acre zoo grounds. Four officers hunted down and shot the animal after police got a 911 call from a zoo employee.

The zoo has a response team that can shoot animals. But zoo officials and police described the initial moments after the escape as chaotic.

The dead man was identified as Carlos Sousa Jr., 17, of San Jose.

The injured men, 19- and 23-year-old brothers from San Jose, were upgraded to stable condition at San Francisco General Hospital after surgery. They suffered deep bites and claw wounds on their heads, necks, arms and hands, said Dr. Rochelle Dicker, a surgeon. She said they were expected to recover fully.

The zoo's director of animal care and conservation, Robert Jenkins, said the tiger did not leave through an open door. "The animal appears to have climbed or otherwise leaped out of the enclosure," he said.

Hanna predicted that other U.S. zoos would reassess their tiger enclosures if it turns out the tiger was able to leap out. He said he never before heard of a zoo visitor being killed by an animal.

"It's much safer going to a zoo than getting in your car and going down the driveway," he said.

Hanna said that because zoo tigers are well-fed, it is unlikely the animal was looking for food when it got out. "Were they taunting the animal?" he said. "Were they throwing things that were making it angry?"

The first attack happened outside the tiger's enclosure, and the victim died at the scene. Another was about 300 yards away, in front of the zoo cafe. About 20 visitors were in the zoo when the attacks happened, officials said.