SAN FRANCISCO &

Investigators trying to determine how a tiger escaped its enclosure at the San Francisco Zoo &

killing one visitor and mauling two others on Christmas &

planned a thorough sweep of the zoo grounds today to look for clues.




Authorities did not believe more people were attacked, but they wanted to inspect the area in the daylight. Zoo officials were still uncertain how long the Siberian tiger, the same one that mauled a zookeeper almost one year earlier, had been loose before being killed by police.




The three men who were attacked while visiting the zoo Tuesday 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 zoo was closed today.




The two injured men were listed in stable condition at San Francisco General Hospital. Dr. John Brown, an emergency room physician, told ABC's "Good Morning America" today they suffered deep bites and claw cuts to their heads, necks, arms and hands.




"These injuries are severe injuries, but they are very treatable, and these two gentlemen seem to be in good health, so I think they have a good chance," Brown 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 approximately 300-pound female 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 are 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 cage's iron bars and grabbed a keeper, biting and mauling one of her 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 a $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.




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.