A tiger that mauled a zookeeper last year escaped from its pen at the San Francisco Zoo on Tuesday, killing one man and injuring two others before police shot it dead, authorities said. The three men were in their 20s; they were together and were not zoo employees, San Francisco Police spokesman Steve Mannina said. They were attacked 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. It was unclear how the tiger escaped or how long it was on the loose. The approximately 300-pound female Siberian tiger, named Tatiana, attacked a zookeeper last December during a public feeding, according to the zoo's director of animal care and conservation. Robert Jenkins, the zoo's director of animal care and conservation, could not explain how the animal 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," said Jenkins. "The animal appears to have climbed or otherwise leapt out of the enclosure." The zoo, which is open 365 days a year, was evacuated immediately after the attack was reported. The two injured men were in critical but stable condition at San Francisco General Hospital, Fire Department spokesman Lt. Ken Smith said. A call to the supervising nurse at San Francisco General was not immediately returned. "This is a tragic event for San Francisco," Smith said. "We pride ourselves in our zoo, and we pride ourselves in tourists coming and looking at our city." Authorities did not believe there were any other people attacked, but because it was dark they could not be certain. Investigators remained on the scene using ladders and flashlights. Smith said a thorough sweep of the grounds would be conducted in the morning. Investigators working to understand what happened have sketched a chilling picture. The first attack happened right outside the Siberian's enclosure - the victim died on the scene. A group of four responding officers came across his body when they made their way into the dark zoo grounds, said Mannina. 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 animal, 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. 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. There are five tigers at the zoo - three Sumatrans, and two Siberians. Officials initially worried that four tigers had escaped, but they soon learned that only one had left its pen, according to Mannina. Last December, the animal reached through the cage's iron bars and badly lacerated a zookeeper's arm. The zoo's Lion House was temporarily closed during an investigation. California's Division of Occupation Safety and Health blamed the zoo for the assault and imposed a $18,000 penalty, the San Francisco Chronicle reported. San Francisco mayor Gavin Newsom said in a statement that he was deeply saddened by the latest incident and said a thorough investigation is under way. Following 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. The zoo will be closed on Wednesday.