San Francisco ? The big cat exhibit at the San Francisco Zoo was cordoned off as a crime scene Wednesday as investigators tried to determine whether a 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 300-pound 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.

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. He said taunting can definitely make an animal more aggressive, 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.