Tatiana the Siberian tiger killed one man and mauled two others before police shot her on Christmas Day. ((San Francisco Zoo/Associated Press))

Officials are trying to figure out how a tiger that killed a 17-year-old boy and injured two othersgot out of its enclosure at the San Francisco Zoo, which police are now treating as a crime scene.

The female Siberian tiger, Tatiana, who last year attacked a zookeeper, escaped on Christmas Day, killing the teen and mauling two others before police shot and killed her.

On Wednesday, theSan Francisco medical examiner's office identified the young victim as Carlos Sousa of San Jose, Calif. The San Francisco Chronicle said the other twowho were attackedwere brothers, and friends of Sousa.

The brothers were upgraded to stable condition Wednesday at San Francisco General Hospital after surgery to have their wounds cleaned and closed, said Dr. Rochelle Dicker, a surgeon. They suffered deep bites and claw cuts on their heads, necks, arms and hands.

The 140-kilogram animal was in an enclosure with a five-metre-wide moat and six-metre-high walls, yet zoo official Robert Jenkins saidthe tigermust have climbed or leaped out of the area.

"There was no way out through the door," he maintained.

But police are viewing the area as a potential crime scene because they don't know whether the tiger escaped through human action or by itself, said San Francisco police Chief Heather Fong.

Officials are also investigating whether the three taunted the tiger.

Police arrived at the zoo shortly after5 p.m. local timeandfoundSousa, mauled to death, near the tiger's pen.

The tiger was about 300 metres away, sitting next to a man who was bleeding from his head, Ocean Beach police spokesman Steve Mannina said.

"The tiger re-engaged the victim. As the officers moved even closer, the tigerfocused its attention on the officers. It started comingtoward the officers. That's when the officers fired."

Last year, Tatiana lashed out at azookeeper, ripping the woman's arm during a public feeding demonstration inside a cage.