The injured victims are described as being in a critical condition Victims reach hospital

Police shot dead the 300lb (136kg) beast, named Tatiana, which mauled a keeper just before Christmas last year.

Zoo officials said they were baffled as to how the Siberian tiger got out at around closing time on Christmas Day.

The two survivors have been upgraded to stable condition at San Francisco General Hospital.

They have undergone surgery after suffering bites and claw cuts to their upper body.

Dr Rochelle Dicker at the hospital said: "Our two victims, I'm happy to report, are doing very well right now. They are in very stable condition; they're in good spirits."

'Aggressive bite marks'

Emergency services were called to the zoo at around 1715 (0115 GMT) after the tiger went on the rampage.

There was no way out through the door. The animal appears to have climbed or otherwise leaped out of the enclosure Robert Jenkins, zoo director

The zoo was evacuated and armed police officers and firefighters sent to the scene.

It was dark by the time police arrived at the zoo and barely a couple of dozen visitors remained.

Three men - one of them 19 years old and the other two in their early 20s - were attacked, suffering "pretty aggressive bite marks", zoo spokesman Steve Mannina told the Associated Press news agency.

Officers found the dead body of one of the victims right outside the tiger's enclosure.

Tatiana attacked one of the zoo's keepers last year

The second victim was found about 300m away, in front of a cafe, sitting on the ground, with blood running from gashes in his head.

Tatiana sat next to him and suddenly attacked him again, Mr Mannina said.

Officers approached, and fired at the animal with .40 calibre handguns when it began to advance towards them.

The third victim was found in the cafe.

Officials were at a loss to explain how Tatiana got out of the enclosure, which is surrounded by a 15ft (4.5m) wide moat and a 20ft (6m) high wall.

The zoo's director of animal care and conservation, Robert Jenkins, said: "There was no way out through the door.

"The animal appears to have climbed or otherwise leaped out of the enclosure."

San Francisco Zoo is home to Siberian and Sumatran tigers.

In December 2006, a keeper at the zoo had her arm severely lacerated when Tatiana reached through the bars of her cage and mauled her during a public feeding.