(CBS/AP) Investigators trying to determine how a tiger escaped its zoo enclosure on Christmas Day - killing one man and mauling two others - plan a thorough sweep of the zoo grounds Wednesday to look for clues.
Authorities do not believe more people were attacked, but they want to inspect the area in the daylight. Zoo officials are still uncertain how long the tiger, which last year badly mauled a zookeeper, was loose before being shot dead.
Neighbors heard the gun shots as police killed the Siberian tiger, reports CBS News correspondent John Blackstone.
The three men who were attacked Tuesday while visiting the zoo were in their 20s, police spokesman Steve Mannina said. The attack occurred just after the 5 p.m. closing time, on the east end of the 125-acre grounds.
They suffered "pretty aggressive bite marks," Mannina said.
The two injured men were listed in critical but stable condition at San Francisco General Hospital. John Brown, an emergency room physician, told the San Francisco Chronicle that they suffered deep bites and claw cuts to their heads and upper bodies.
The Siberian tiger, named Tatiana, was the same giant cat that attacked a zookeeper just about a year ago during a public feeding, said Robert Jenkins, the zoo's director of animal care and conservation.
The approximately 300-pound female did not leave through an open door, Jenkins said. But he could not explain how it escaped - the tiger's enclosure is surrounded by a 15-foot-wide moat and 20-foot-high walls.
"There was no way out through the door," Jenkins said. "The animal appears to have climbed or otherwise leapt out of the enclosure."
The first attack happened right outside the Siberian's enclosure - the victim died at the scene. A group of four responding officers came across his body when they made their way into the dark zoo grounds, Mannina said.
Then they saw the second victim. He 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. Tatiana sat next to him. Suddenly, the cat attacked the man again, Mannina said.
The officers started approaching the tiger, bearing their handguns. Tatiana started moving in their direction. Several of the officers then fired, killing the animal.
Only then did they see the third victim, who had also been mauled.
"They look like cute and cuddly animals, but they are predators," animal trainer Chris Austria told The Early Show. "Even if they're raised in captivity, they still have that aggressive nature."
The zoo is open 365 days a year. Although no new visitors were let in after 5 p.m., the grounds were not scheduled to close until an hour later, and there were between 20 and 25 people still on site 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 are five tigers at the zoo - three Sumatrans and two Siberians. Officials initially worried that four tigers had escaped, but soon learned only one had escaped its pen, Mannina said.
On Dec. 22, 2006, Tatiana reached through the cage's iron bars and attacked a keeper with her claws and teeth, causing deep lacerations to the worker's arms. The zoo's Lion House was temporarily closed during an investigation.
"The thing about tigers, when they escape and they're outside of their environment, outside an area where they're not comfortable, they tend to be very defensive and very threatened," Austria told The Early Show.
California's Division of Occupational Safety and Health blamed the zoo for the assault and imposed a $18,000 penalty. A medical claim filed against the city by the keeper was denied.
Mayor Gavin Newsom said in a statement he was deeply saddened by the latest attack and that a thorough investigation was under way.
After last year's attack, the zoo added customized steel mesh over the bars, built in a feeding shoot 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 zoo will be closed Wednesday.
© MMVII, CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.
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