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 teenage visitor Christmas Day escaped from her high-walled pen on her own or got help from someone, inadvertent or otherwise.

Police shot and killed the tiger, 4-year-old Tatiana, after a rampage that began when she escaped from an enclosure surrounded by what zoo officials said are an 18-foot wall and a 20-foot moat. Two other visitors, brothers ages 19 and 23, were severely mauled.

Police Chief Heather Fong said the department 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 had 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 enclosure as well as accounts from witnesses and others.

One zoo official insisted that Tatiana, one of five tigers at the San Francisco site, did not get out through an open door and must have climbed or leaped out.

Jack Hanna, former director of the Columbus Zoo and a frequent TV guest, said such a leap would be an unbelievable feat and "virtually impossible."

He speculated that visitors might have been fooling around and might have taunted the tiger and perhaps even helped her get out by, say, putting a board in the moat.

The police chief would not comment on whether the tiger had been taunted.

Just before Christmas a year ago, Tatiana ripped the flesh off a zookeeper's arm while the woman was feeding Tatiana 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 gave no thought to destroying Tatiana after the 2006 incident, because she "was acting as a normal tiger does." As for whether Tatiana showed any warning signs before Tuesday's attack, he said: "She seemed to be very well-adjusted into that exhibit."

The three visitors were attacked around closing time Tuesday on the 125-acre zoo grounds. Four officers hunted down and shot Tatiana 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 visitor was identified as Carlos Sousa Jr., 17, of San Jose. The injured men, whose names were not disclosed but who are also 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 surgeon Rochelle Dicker, who said she expected them to make a full recovery.

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

Mollinedo, the zoo director, said, "We're still not too clear as to exactly what transpired."

The fatal attack happened outside Tatiana's enclosure. Another attack took place about 300 yards away, in front of the zoo cafe. The police chief said that the tiger was mauling one of the men, and that when officers yelled at her to stop, she turned toward them and they opened fire. Only then did they see the third victim, police said.

About 20 visitors were in the zoo at the time, officials said. Employees and visitors were told to take shelter, and some employees locked themselves inside buildings.

The zoo was closed yesterday. Officials said they expected to reopen today but said the big-cat exhibit would stay closed "until we get a better understanding of what actually happened," Mollinedo said.

After last year's attack, the state fined the zoo $18,000. The zoo added steel mesh over the bars, built in a feeding chute, and increased the distance between the public and the cats.

Spokesman Jim Rogers said the U.S. Department of Agriculture was looking into the attack for violations of animal-welfare laws.